Kernel development

Release status

Kernel release status

The 2.6.26 merge window is open, so there is no current 2.6 development release. See the article below for a summary of the patches merged for 2.6.26 so far.

The current -mm tree is 2.6.25-mm1. Recent changes to -mm include some read-copy-update enhancements and the OLPC architecture support code, but mostly -mm is just getting ready for the big flow of patches into the mainline. See the -mm merge plans document for Andrew's plans for 2.6.26.

The current stable 2.6 kernel is 2.6.25, released on April 16. After nearly three months of development and the merging of over 12,000 patches from almost 1200 developers, this kernel is now considered ready for wider use. Highlights of this release include the ath5k (Atheros wireless) driver, a bunch of realtime work including realtime group scheduling, preemptable RCU, LatencyTop support, a number of new ext4 filesystem features, support for the controller area network protocol, more network namespace work, the return of the timerfd() system call, the page map patches (providing much better information on system memory use), the SMACK security module, better kernel support for Intel and ATI R500 graphics chipsets, the memory use controller, ACPI thermal regulation support, MN10300 architecture support, and much more. See the KernelNewbies 2.6.25 page for lots of details, or the full changelog for unbelievable amounts of detail.

2.6.24.5 was released on April 18. It contains a relatively long list of fixes for significant 2.6.24 problems.

For older kernels: 2.4.36.3 was released on April 19. "Nothing outstanding here, I've just decided to release pending fixes. Those already running 2.4.36.2 have no particular reason to upgrade, unless they already experience troubles in the fixed areas."

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Kernel development news

Quotes of the week

-- Keith Packard

-- Andrew Morton

-- Ingo Molnar

Comments (none posted)

The 2.6.26 merge window opens

That shiny new 2.6.25 kernel which was released on April 16 is now ancient history; some 3500 changesets have been merged into the mainline git repository since then. Some of the most significant user-visible changes include:

• New drivers for Korina (IDT rc32434) Ethernet MACs, SuperH MX-G and SH-MobileR2 CPUs, Solution Engine SH7721 boards, ARM YL9200, Kwikbyte KB9260, Olimex SAM9-L9260, and emQbit ECB_AT91 boards, Digi ns921x processors, the Nias Digital SMX crypto engines, AMCC PPC460EX evaluation boards, Emerson KSI8560 boards, Wind River SBC8641D boards, Logitech Rumblepad 2 force-feedback devices, Renesas SH7760 I2C controllers, and SuperH Mobile I2C controllers.

• The PCI subsystem now supports PCI Express Active State Power Management, which can yield significant power savings on suitably equipped hardware.

• There is a new security= boot parameter which allows the specification of which security module to use if more than one are available.

• Network address translation (NAT) is now supported for the SCTP, DCCP, and UDP-Lite protocols. There is also netfilter connection tracking support for DCCP.

• The network stack can now negotiate selective acknowledgments and window scaling even when syncookies are in use.

• Another long series of network namespace patches has been merged, continuing the long process of making all networking code namespace-aware.

• Mesh networking support has been added to the mac80211 layer. It is currently marked "broken," though, until various outstanding issues are fixed.

• 4K stacks are now the default for the x86 architecture. This change is controversial and could be reversed by the time the final release happens.

• SELinux now supports "permissive types" which allow specific domains to run as if SELinux were not present in the system at all.

• A number of enhancements have been made to the realtime group scheduler, including multi-level groups, the ability to mix processes and groups (and have them compete against each other for CPU time), better SMP balancing, and more.

• Support for the running of SunOS and Solaris binaries has been removed; it has long been unmaintained and did not work well.

• The kernel now has support for read-only bind mounts, which provide a read-only view into an otherwise writable filesystem. This feature (the implementation of which was more involved than one might think) is intended for use in containers and other situations where even processes running as root should not be able to modify certain filesystems.

Changes visible to kernel developers include:

• At long last, support for the KGDB interactive debugger has been added to the x86 architecture. There is a DocBook document in the Documentation directory which provides an overview on how to use this new facility.

• Page attribute table (PAT) support is also (again, at long last) available for the x86 architecture. PATs allow for fine-grained control of memory caching behavior with more flexibility than the older MTRR feature. See Documentation/x86/pat.txt for more information.

• Two new functions (inode_getsecid() and ipc_getsecid()), added to support security modules and the audit code, provide general access to security IDs associated with inodes and IPC objects. A number of superblock-related LSM callbacks now take a struct path pointer instead of struct nameidata. There is also a new set of hooks providing generic audit support in the security module framework.

• The now-unused ieee80211 software MAC layer has been removed; all of the drivers which needed it have been converted to mac80211. Also removed are the sk98lin network driver (in favor of skge) and bcm43xx (replaced by b43 and b43legacy).

• The generic semaphores patch has been merged. The semaphore code also has new down_killable() and down_timeout() functions.

• The ata_port_operations structure used by libata drivers now supports a simple sort of operation inheritance, making it easier to write drivers which are "almost like" existing code, but with small differences.

• A new function (ns_to_ktime()) converts a time value in nanoseconds to ktime_t.

• The final users of struct class_device have been converted to use struct device instead. If all goes well, the class_device structure will be removed later in the 2.6.26 cycle.

• Greg Kroah-Hartman is no longer the PCI subsystem maintainer, having passed that responsibility on to Jesse Barnes.

• The seq_file code now accepts a return value of SEQ_SKIP from the show() callback; that value causes any accumulated output from that call to be discarded.

Needless to say, this development series is still young and, as of this writing, the merge window has over a week to run. So there will be a lot more code going into the mainline before the shape of 2.6.26 becomes clear.

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4K stacks by default?

The kernel stack is a rather important chunk of memory in any Linux system. The unpleasant kernel memory corruption that results from overflowing it is something that is to be avoided at all costs. But the stack is allocated for each process and thread in the system, so those who are looking to reduce memory usage target the 8K stack used by default on x86. In addition, an 8K stack requires two physically contiguous pages (an "order 1" allocation) which can be difficult to satisfy on a running system due to fragmentation.

Linux has had optional support for 4K stacks for nearly four years now, with Fedora and RHEL enabling it on the kernels they ship, but a recent patch to make it the default for x86 has raised some eyebrows. Andrew Morton sees it as bypassing the normal patch submission process:

It is not surprising that patch author Ingo Molnar sees things a little differently:

As described in an earlier LWN article, the main concerns about only providing 4K for the kernel stack are for complicated storage configurations or for people using NDISwrapper. There is fairly high disdain for the latter case—as it is done to load proprietary Windows drivers into the kernel—but it could lead to a pretty hideous failure in the former. Data corruption certainly seems like a possibility, but, regardless, a kernel crash is definitely not what an administrator wants to have to deal with.

Arjan van de Ven summarized the current state, noting that NDISwrapper really requires 12K stacks, so having 8K only makes it less likely those kernels will crash. The stacking of multiple storage drivers (network filesystems, device mapper, RAID, etc.) is a bigger issue:

Proponents of default 4K stacks seem to be puzzled why there is objection to the change since there have been no problems with Red Hat kernels. But Andi Kleen notes:

The xfs filesystem, which is not supported in RHEL or Fedora, can potentially use a great deal of stack. This leads some kernel hackers to worry that a complicated configuration that uses it, an "nfs+xfs+md+scsi writeback" configuration as Eric Sandeen puts it, could overflow. Work is already proceeding to reduce the xfs stack usage, but it clearly is a problem that xfs hackers have seen. David Chinner responds to a question about stack overflows:

It would seem premature to make 4K stacks the default. There is good reason to believe that folks using xfs could run into problems. But there is a larger issue, one that Morton brought up in his initial message, then reiterated later in the thread:

The memory savings can be significant, especially in the embedded world. Coupled with the elimination of order 1 allocations each time a process gets created, there is good reason to keep working toward 4K stacks by default. As of this writing, the default remains for 4K stacks in Linus's tree, but that could change before long.

Comments (25 posted)

ELC: Morton and Saxena on working with the kernel community

In many ways, Andrew Morton's keynote set the tone for this year's Embedded Linux Conference (ELC) by describing the ways that embedded companies and developers can work with the kernel community in a way that will be "mutually beneficial". Morton provided reasons, from a purely economic standpoint, why it makes sense for companies to get their code into the mainline kernel. He also provided concrete suggestions on how to make that happen. The theme of the conference seemed to be "working with the community" and Morton's speech provided an excellent example of how and why to do just that.

Conference organizer Tim Bird introduced the keynote as "the main event" for ELC, noting that he often thought of Morton as "kind of like the adult in the room" on linux-kernel. Readers of that mailing list tend to get the impression that there's more than one of him around because of all that he does. He also noted that it was surprising to some that Morton has an embedded Linux background—from his work at Digeo.

Morton believes that embedded development is underrepresented in kernel.org work relative to its economic importance. This is caused by a number of factors, not least the financial constraints under which much embedded development is done. An exceptional case is the chip and board manufacturers who have a big interest in seeing Linux run well on their hardware so that they can attract more customers. But even those do not contribute as much as he would like to see to kernel development.

An effect of this underrepresentation is a risk that it will tilt kernel development more toward the server and desktop. The kernel team is already accused of being server-centric, and there is some truth to that, "but not as much as one might think". Kernel hackers do care about the desktop as well as embedded devices, but without an advocate for embedded concerns, sometimes things get missed.

Something Morton would like to see is a single full-time "embedded maintainer". That person would serve as the advocate for embedded concerns, ensuring that they didn't get overlooked in the process. An embedded maintainer could make a significant impact for embedded development.

Not all kernel contributions need to be code, he said. There is a need just to hear the problems that are being faced by the embedded community along with lists of things that are missing. "Senior, sophisticated people" are needed to help prioritize the features that are being considered as well. Morton often finds out things he didn't know at conferences, things that he should have known about much earlier: "That's bad!"

Morton is trying to incite the embedded community to interact with the kernel hackers more on linux-kernel. He said that a great way to get the attention of the team is to come onto the mailing list and make them look bad. Unfavorable comparisons to other systems or earlier kernels, for example, especially when backed up with numbers, are noticed quickly. He said that it is important to remember that the person who makes the most noise gets the most attention.

One of the areas that he is most concerned about is the practice of "patch hoarding"—holding on to kernel changes as patches without submitting them upstream to the kernel hackers. It is hopefully only due to a lack of resources, but he has heard that some are doing it to try and gain a competitive advantage. This is simply wrong, he said, companies have a "moral if not legal obligation" to submit those patches.

[PULL QUOTE: The code will be better because of the review done by the kernel hackers; once it is done, the maintenance cost falls to near zero as well. He also touted the competitive advantage, noting that getting your code merged means that you have won—competing proposals won't get in. END QUOTE]

There are many good reasons for getting code merged upstream that Morton outlined. The code will be better because of the review done by the kernel hackers; once it is done, the maintenance cost falls to near zero as well. He also touted the competitive advantage, noting that getting your code merged means that you have won—competing proposals won't get in. Being the first to merge a feature can make it easier on yourself and harder on your competition.

There are downsides to getting your code upstream as well. Most of those stem from not getting code out there early enough for review. The kernel developers can ask for significant changes to the code especially in the area of user space interfaces. If a company already has lots of code using the new feature and/or interface, it could be very disruptive; "sorry, there's no real fix for that except getting your code out early enough".

Another downside that companies may run into is with competitors being brought into the process. Morton and other kernel hackers will try to find others who might have a stake in a new feature to get them involved so that everybody's needs are taken into account. This can blunt the "win" of getting your feature merged. Some are also concerned that competitors will get access to the code once it has been submitted; "tough luck" Morton said, everything in the kernel is GPL.

Morton had specific suggestions for choosing a kernel version to use for an embedded project. 2.6.24 is not a lot better than 2.4.18 for embedded use, but it has one important feature: the kernel team will be interested in bugs in the current kernel. He suggests starting with the current kernel, upgrading it while development proceeds, freezing it only when it is time to ship the product.

He also suggests that a company create an internal kernel team with one or two people who are the interface to linux-kernel. This will help with name recognition on the mailing list, which will in turn get patches submitted more attention. Over time, by participating and reviewing others' code, the interface people will build up "brownie points" that will allow them to call in favors to get their code reviewed, or to help smooth the path for inclusion.

The kernel.org developers appear to give free support, generally very good support, Morton said, but it is not truly free. Kernel hackers do it as a "mutually beneficial transaction"; they don't do it to make more money for your company, they do it to make the kernel better. Morton is definitely a big part of that, inviting people to email him, especially if "five minutes of my time can save months of yours".

The decision about when to merge a new feature is hard for some to understand. Many consider Linux a dictatorship, which is incorrect, it is instead "a democracy that doesn't vote". The merge decision is made on the model of the "rule of law" with kernel hackers playing the role of judges. Unfortunately, there are few written rules.

Some of the factors that go into his decision about a particular feature are its maintainability, whether there will be an ongoing maintenance team, as well as the general usefulness of the feature. Depending on the size of the feature, an ongoing maintenance team can be the deciding factor. It is not so important for a driver, but a new architecture, for example, needs ongoing maintenance that can only be done by people with knowledge of and access to the hardware.

MontaVista kernel hacker, Deepak Saxena, gave a presentation entitled "Appropriate Community Practices: Social and Technical Advice" later in the conference that mirrored many of Morton's points. He showed some examples of hardware vendors making bad decisions that got shot down by the kernel developers, mostly because they didn't "release early and release often". There is a dangerous attitude that "it's Linux, it's open source, I can do anything I want" which is true, but won't get you far with the community.

Saxena has high regard for the benefits of working with the system: if your competitor is active in the community, they are getting an advantage that you aren't. Like Morton, he believes that some members of the development team need to get involved in kernel.org activities. "The community is an extension of your team, your team is an extension of the community."

He also has specific advice for hardware vendors: avoid abstraction layers, recognize that your hardware is not unique, and think beyond the reference board implementation. Generalizing your code so that others can use it will make it much more acceptable, as will talking with the developers responsible for the subsystems you are touching. Abstraction layers may be helpful for hardware vendors trying to support multiple operating systems, but they make it difficult for the kernel hackers to understand and maintain the code. The kernel.org folks are not interested in finding and fixing bugs in an abstraction layer.

He also points out additional benefits of getting code merged. Once it is in the kernel, the company's team will no longer have to keep up with kernel releases, updating their patches to follow the latest changes. The code will still need to be maintained, but day-to-day changes will be handled by the kernel.org folks. An additional benefit is that the code will be enhanced by various efforts to automatically find bugs in mainline kernel code with tools like lockdep.

It is clear that the kernel hackers are making a big effort to not only get code from the embedded folks, but also some of their expertise. There are various outreach efforts to try and get more people involved in the Linux development process; these two talks are certainly a part of that. By making it clear that there are benefits to both parties, they hope to make an argument that will reach up from engineering to management resulting in a better kernel for all.

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Integrating and Validating dynticks and Preemptable RCU

Introduction

Read-copy update (RCU) is a synchronization mechanism that was added to the Linux kernel in October of 2002. RCU is most frequently described as a replacement for reader-writer locking, but it has also been used in a number of other ways. RCU is notable in that RCU readers do not directly synchronize with RCU updaters, which makes RCU read paths extremely fast, and also permits RCU readers to accomplish useful work even when running concurrently with RCU updaters.

In early 2008, a preemptable variant of RCU was accepted into mainline Linux in support of real-time workloads, a variant similar to the RCU implementations in the -rt patchset since August 2005. Preemptable RCU is needed for real-time workloads because older RCU implementations disable preemption across RCU read-side critical sections, resulting in excessive real-time latencies.

However, one disadvantage of the -rt implementation was that each grace period required work to be done on each CPU, even if that CPU is in a low-power “dynticks-idle” state, and thus incapable of executing RCU read-side critical sections. The idea behind the dynticks-idle state is that idle CPUs should be physically powered down in order to conserve energy. In short, preemptable RCU can disable a valuable energy-conservation feature of recent Linux kernels. Although Josh Triplett and Paul McKenney had discussed some approaches for allowing CPUs to remain in low-power state throughout an RCU grace period (thus preserving the Linux kernel's ability to conserve energy), matters did not come to a head until Steve Rostedt integrated a new dyntick implementation with preemptable RCU in the -rt patchset.

This combination caused one of Steve's systems to hang on boot, so in October, Paul coded up a dynticks-friendly modification to preemptable RCU's grace-period processing. Steve coded up rcu_irq_enter() and rcu_irq_exit() interfaces called from the irq_enter() and irq_exit() interrupt entry/exit functions. These rcu_irq_enter() and rcu_irq_exit() functions are needed to allow RCU to reliably handle situations where a dynticks-idle CPUs is momentarily powered up for an interrupt handler containing RCU read-side critical sections. With these changes in place, Steve's system booted reliably, but Paul continued inspecting the code periodically on the assumption that we could not possibly have gotten the code right on the first try.

Paul reviewed the code repeatedly from October 2007 to February 2008, and almost always found at least one bug. In one case, Paul even coded and tested a fix before realizing that the bug was illusory, but in all cases, the “bug” was in fact illusory.

Near the end of February, Paul grew tired of this game. He therefore decided to enlist the aid of Promela and spin, as described in the LWN article Using Promela and Spin to verify parallel algorithms. This article presents a series of seven increasingly realistic Promela models, the last of which passes, consuming about 40GB of main memory for the state space.

More important, Promela and Spin did find a very subtle bug for me!!!

This article is organized as follows:

1. Introduction to Preemptable RCU and dynticks
   1. Task Interface
   2. Interrupt Interface
   3. Grace-Period Interface
2. Validating Preemptable RCU and dynticks
   1. Basic Model
   2. Validating Safety
   3. Validating Liveness
   4. Interrupts
   5. Validating Interrupt Handlers
   6. Validating Nested Interrupt Handlers
   7. Validating NMI Handlers

These sections are followed by conclusions and answers to the Quick Quizzes.

Introduction to Preemptable RCU and dynticks

The per-CPU dynticks_progress_counter variable is central to the interface between dynticks and preemptable RCU. This variable has an even value whenever the corresponding CPU is in dynticks-idle mode, and an odd value otherwise. A CPU exits dynticks-idle mode for the following three reasons:

1. to start running a task,
2. when entering the outermost of a possibly nested set of interrupt handlers, and
3. when entering an NMI handler.

Preemptable RCU's grace-period machinery samples the value of the dynticks_progress_counter variable in order to determine when a dynticks-idle CPU may safely be ignored.

The following three sections give an overview of the task interface, the interrupt/NMI interface, and the use of the dynticks_progress_counter variable by the grace-period machinery.

Task Interface

When a given CPU enters dynticks-idle mode because it has no more tasks to run, it invokes rcu_enter_nohz():

  1 static inline void rcu_enter_nohz(void)
  2 {
  3   mb();
  4   __get_cpu_var(dynticks_progress_counter)++;
  5   WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1);
  6 }

This function simply increments dynticks_progress_counter and checks that the result is even, but first executing a memory barrier to ensure that any other CPU that sees the new value of dynticks_progress_counter will also see the completion of any prior RCU read-side critical sections.

Similarly, when a CPU that is in dynticks-idle mode prepares to start executing a newly runnable task, it invokes rcu_exit_nohz:

  1 static inline void rcu_exit_nohz(void)
  2 {
  3   __get_cpu_var(dynticks_progress_counter)++;
  4   mb();
  5   WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
  6 }

This function again increments dynticks_progress_counter, but follows it with a memory barrier to ensure that if any other CPU sees the result of any subsequent RCU read-side critical section, then that other CPU will also see the incremented value of dynticks_progress_counter. Finally, rcu_exit_nohz() checks that the result of the increment is an odd value.

The rcu_enter_nohz() and rcu_exit_nohz functions handle the case where a CPU enters and exits dynticks-idle mode due to task execution, but does not handle interrupts, which are covered in the following section.

Interrupt Interface

The rcu_irq_enter() and rcu_irq_exit() functions handle interrupt/NMI entry and exit, respectively. Of course, nested interrupts must also be properly accounted for. The possibility of nested interrupts is handled by a second per-CPU variable, rcu_update_flag, which is incremented upon entry to an interrupt or NMI handler (in rcu_irq_enter()) and is decremented upon exit (in rcu_irq_exit()). In addition, the pre-existing in_interrupt() primitive is used to distinguish between an outermost or a nested interrupt/NMI.

Interrupt entry is handled by the rcu_irq_enter shown below:

  1 void rcu_irq_enter(void)
  2 {
  3   int cpu = smp_processor_id();
  4 
  5   if (per_cpu(rcu_update_flag, cpu))
  6     per_cpu(rcu_update_flag, cpu)++;
  7   if (!in_interrupt() &&
  8       (per_cpu(dynticks_progress_counter, cpu) & 0x1) == 0) {
  9     per_cpu(dynticks_progress_counter, cpu)++;
 10     smp_mb();
 11     per_cpu(rcu_update_flag, cpu)++;
 12   }
 13 }

Line 3 fetches the current CPU's number, while lines 4 and 5 increment the rcu_update_flag nesting counter if it is already non-zero. Lines 6 and 7 check to see whether we are the outermost level of interrupt, and, if so, whether dynticks_progress_counter needs to be incremented. If so, line 9 increments dynticks_progress_counter, line 10 executes a memory barrier, and line 11 increments rcu_update_flag. As with rcu_exit_nohz(), the memory barrier ensures that any other CPU that sees the effects of an RCU read-side critical section in the interrupt handler (following the rcu_irq_enter() invocation) will also see the increment of dynticks_progress_counter.

Interrupt entry is handled similarly by rcu_irq_exit():

  1 void rcu_irq_exit(void)
  2 {
  3   int cpu = smp_processor_id();
  4 
  5   if (per_cpu(rcu_update_flag, cpu)) {
  6     if (--per_cpu(rcu_update_flag, cpu))
  7       return;
  8     WARN_ON(in_interrupt());
  9     smp_mb();
 10     per_cpu(dynticks_progress_counter, cpu)++;
 11     WARN_ON(per_cpu(dynticks_progress_counter, cpu) & 0x1);
 12   }
 13 }

Line 3 fetches the current CPU's number, as before. Line 5 checks to see if the rcu_update_flag is non-zero, returning immediately (via falling off the end of the function) if not. Otherwise, lines 6 through 11 come into play. Line 6 decrements rcu_update_flag, returning if the result is not zero. Line 8 verifies that we are indeed leaving the outermost level of nested interrupts, line 9 executes a memory barrier, line 10 increments dynticks_progress_counter, and line 11 verifies that this variable is now even. As with rcu_enter_nohz(), the memory barrier ensures that any other CPU that sees the increment of dynticks_progress_counter will also see the effects of an RCU read-side critical section in the interrupt handler (preceding the rcu_irq_enter() invocation).

These two sections have described how the dynticks_progress_counter variable is maintained during entry to and exit from dynticks-idle mode, both by tasks and by interrupts and NMIs. The following section describes how this variable is used by preemptable RCU's grace-period machinery.

Grace-Period Interface

Of the four preemptable RCU grace-period states shown below (taken from The Design of Preemptable Read-Copy Update), only the rcu_try_flip_waitack_state() and rcu_try_flip_waitmb_state() states need to wait for other CPUs to respond.

Of course, if a given CPU is in dynticks-idle state, we shouldn't wait for it. Therefore, just before entering one of these two states, the preceding state takes a snapshot of each CPU's dynticks_progress_counter variable, placing the snapshot in another per-CPU variable, rcu_dyntick_snapshot. This is accomplished by invoking dyntick_save_progress_counter, shown below:

  1 static void dyntick_save_progress_counter(int cpu)
  2 {
  3   per_cpu(rcu_dyntick_snapshot, cpu) =
  4     per_cpu(dynticks_progress_counter, cpu);
  5 }

The rcu_try_flip_waitack_state() state invokes rcu_try_flip_waitack_needed(), shown below:

  1 static inline int
  2 rcu_try_flip_waitack_needed(int cpu)
  3 {
  4   long curr;
  5   long snap;
  6 
  7   curr = per_cpu(dynticks_progress_counter, cpu);
  8   snap = per_cpu(rcu_dyntick_snapshot, cpu);
  9   smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
 10   if ((curr == snap) && ((curr & 0x1) == 0))
 11     return 0;
 12   if ((curr - snap) > 2 || (snap & 0x1) == 0)
 13     return 0;
 14   return 1;
 15 }

Lines 7 and 8 pick up current and snapshot versions of dynticks_progress_counter, respectively. The memory barrier on line ensures that the counter checks in the later rcu_try_flip_waitzero_state follow the fetches of these counters. Lines 10 and 11 return zero (meaning no communication with the specified CPU is required) if that CPU has remained in dynticks-idle state since the time that the snapshot was taken. Similarly, lines 12 and 13 return zero if that CPU was initially in dynticks-idle state or if it has completely passed through a dynticks-idle state. In both these cases, there is no way that that CPU could have retained the old value of the grace-period counter. If neither of these conditions hold, line 14 returns one, meaning that the CPU needs to explicitly respond.

For its part, the rcu_try_flip_waitmb_state state invokes rcu_try_flip_waitmb_needed(), shown below:

  1 static inline int
  2 rcu_try_flip_waitmb_needed(int cpu)
  3 {
  4   long curr;
  5   long snap;
  6 
  7   curr = per_cpu(dynticks_progress_counter, cpu);
  8   snap = per_cpu(rcu_dyntick_snapshot, cpu);
  9   smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
 10   if ((curr == snap) && ((curr & 0x1) == 0))
 11     return 0;
 12   if (curr != snap)
 13     return 0;
 14   return 1;
 15 }

This is quite similar to rcu_try_flip_waitack_needed, the difference being in lines 12 and 13, because any transition either to or from dynticks-idle state executes the memory barrier needed by the rcu_try_flip_waitmb_state() state.

We now have seen all the code involved in the interface between RCU and the dynticks-idle state. The next section builds up the Promela model used to validate this code.

Validating Preemptable RCU and dynticks

This section develops a Promela model for the interface between dynticks and RCU step by step, with each of the following sections illustrating one step, starting with the process-level code, adding assertions, interrupts, and finally NMIs.

Basic Model

This section translates the process-level dynticks entry/exit code and the grace-period processing into Promela. We start with rcu_exit_nohz() and rcu_enter_nohz() from the 2.6.25-rc4 kernel, placing these in a single Promela process that models exiting and entering dynticks-idle mode in a loop as follows:

  1 proctype dyntick_nohz()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5 
  6   do
  7   :: i >= MAX_DYNTICK_LOOP_NOHZ -> break;
  8   :: i < MAX_DYNTICK_LOOP_NOHZ ->
  9     tmp = dynticks_progress_counter;
 10     atomic {
 11       dynticks_progress_counter = tmp + 1;
 12       assert((dynticks_progress_counter & 1) == 1);
 13     }
 14     tmp = dynticks_progress_counter;
 15     atomic {
 16       dynticks_progress_counter = tmp + 1;
 17       assert((dynticks_progress_counter & 1) == 0);
 18     }
 19     i++;
 20   od;
 21 }

Lines 6 and 20 define a loop. Line 7 exits the loop once the loop counter i has exceeded the limit MAX_DYNTICK_LOOP_NOHZ. Line 8 tells the loop construct to execute lines 9-19 for each pass through the loop. Because the conditionals on lines 7 and 8 are exclusive of each other, the normal Promela random selection of true conditions is disabled. Lines 9 and 11 model rcu_exit_nohz()'s non-atomic increment of dynticks_progress_counter, while line 12 models the WARN_ON(). The atomic construct simply reduces the Promela state space, given that the WARN_ON() is not strictly speaking part of the algorithm. Lines 14-18 similarly models the increment and WARN_ON() for rcu_enter_nohz(). Finally, line 19 increments the loop counter.

Each pass through the loop therefore models a CPU exiting dynticks-idle mode (for example, starting to execute a task), then re-entering dynticks-idle mode (for example, that same task blocking).

The next step is to model the interface to RCU's grace-period processing. For this, we need to model dyntick_save_progress_counter(), rcu_try_flip_waitack_needed(), rcu_try_flip_waitmb_needed(), as well as portions of rcu_try_flip_waitack() and rcu_try_flip_waitmb(), all from the 2.6.25-rc4 kernel. The following grace_period() Promela process models these functions as they would be invoked during a single pass through preemptable RCU's grace-period processing.

  1 proctype grace_period()
  2 {
  3   byte curr;
  4   byte snap;
  5 
  6   atomic {
  7     printf("MAX_DYNTICK_LOOP_NOHZ = %d\n", MAX_DYNTICK_LOOP_NOHZ);
  8     snap = dynticks_progress_counter;
  9   }
 10   do
 11   :: 1 ->
 12     atomic {
 13       curr = dynticks_progress_counter;
 14       if
 15       :: (curr == snap) && ((curr & 1) == 0) ->
 16         break;
 17       :: (curr - snap) > 2 || (snap & 1) == 0 ->
 18         break;
 19       :: 1 -> skip;
 20       fi;
 21     }
 22   od;
 23   snap = dynticks_progress_counter;
 24   do
 25   :: 1 ->
 26     atomic {
 27       curr = dynticks_progress_counter;
 28       if
 29       :: (curr == snap) && ((curr & 1) == 0) ->
 30         break;
 31       :: (curr != snap) ->
 32         break;
 33       :: 1 -> skip;
 34       fi;
 35     }
 36   od;
 37 }

Lines 6-9 print out the loop limit (but only into the .trail file in case of error) and model a line of code from rcu_try_flip_idle() and its call to dyntick_save_progress_counter(), which takes a snapshot of the current CPU's dynticks_progress_counter variable. These two lines are executed atomically to reduce state space.

Lines 10-22 model the relevant code in rcu_try_flip_waitack() and its call to rcu_try_flip_waitack_needed(). This loop is modeling the grace-period state machine waiting for a counter-flip acknowledgment from each CPU, but only that part that interacts with dynticks-idle CPUs.

Line 23 models a line from rcu_try_flip_waitzero() and its call to dyntick_save_progress_counter(), again taking a snapshot of the CPU's dynticks_progress_counter variable.

Finally, lines 24-36 model the relevant code in rcu_try_flip_waitack() and its call to rcu_try_flip_waitack_needed(). This loop is modeling the grace-period state-machine waiting for each CPU to execute a memory barrier, but again only that part that interacts with dynticks-idle CPUs.

The resulting model, when run with the runspin.sh script, generates 691 states and passes without errors, which is not at all surprising given that it completely lacks the assertions that could find failures. The next section therefore adds safety assertions.

Validating Safety

A safe RCU implementation must never permit a grace period to complete before the completion of any RCU readers that started before the start of the grace period. This is modeled by a grace_period_state variable that can take on three states as follows:

  1 #define GP_IDLE    0
  2 #define GP_WAITING  1
  3 #define GP_DONE    2
  4 byte grace_period_state = GP_DONE;

The grace_period() process sets this variable as it progresses through the grace-period phases, as shown below:

  1 proctype grace_period()
  2 {
  3   byte curr;
  4   byte snap;
  5 
  6   grace_period_state = GP_IDLE;
  7   atomic {
  8     printf("MAX_DYNTICK_LOOP_NOHZ = %d\n", MAX_DYNTICK_LOOP_NOHZ);
  9     snap = dynticks_progress_counter;
 10     grace_period_state = GP_WAITING;
 11   }
 12   do
 13   :: 1 ->
 14     atomic {
 15       curr = dynticks_progress_counter;
 16       if
 17       :: (curr == snap) && ((curr & 1) == 0) ->
 18         break;
 19       :: (curr - snap) > 2 || (snap & 1) == 0 ->
 20         break;
 21       :: 1 -> skip;
 22       fi;
 23     }
 24   od;
 25   grace_period_state = GP_DONE;
 26   grace_period_state = GP_IDLE;
 27   atomic {
 28     snap = dynticks_progress_counter;
 29     grace_period_state = GP_WAITING;
 30   }
 31   do
 32   :: 1 ->
 33     atomic {
 34       curr = dynticks_progress_counter;
 35       if
 36       :: (curr == snap) && ((curr & 1) == 0) ->
 37         break;
 38       :: (curr != snap) ->
 39         break;
 40       :: 1 -> skip;
 41       fi;
 42     }
 43   od;
 44   grace_period_state = GP_DONE;
 45 }

Lines 6, 10, 25, 26, 29, and 44 update this variable (combining atomically with algorithmic operations where feasible) to allow the dyntick_nohz() process to validate the basic RCU safety property. The form of this validation is to assert that the value of the grace_period_state variable cannot jump from GP_IDLE to GP_DONE during a time period over which RCU readers could plausibly persist.

The dyntick_nohz() Promela process implements this validation as shown below:

  1 proctype dyntick_nohz()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5   bit old_gp_idle;
  6 
  7   do
  8   :: i >= MAX_DYNTICK_LOOP_NOHZ -> break;
  9   :: i < MAX_DYNTICK_LOOP_NOHZ ->
 10     tmp = dynticks_progress_counter;
 11     atomic {
 12       dynticks_progress_counter = tmp + 1;
 13       old_gp_idle = (grace_period_state == GP_IDLE);
 14       assert((dynticks_progress_counter & 1) == 1);
 15     }
 16     atomic {
 17       tmp = dynticks_progress_counter;
 18       assert(!old_gp_idle || grace_period_state != GP_DONE);
 19     }
 20     atomic {
 21       dynticks_progress_counter = tmp + 1;
 22       assert((dynticks_progress_counter & 1) == 0);
 23     }
 24     i++;
 25   od;
 26 }

Line 13 sets a new old_gp_idle flag if the value of the grace_period_state variable is GP_IDLE at the beginning of task execution, and the assertion at line 18 fires if the grace_period_state variable has advanced to GP_DONE during task execution, which would be illegal given that a single RCU read-side critical section could span the entire intervening time period.

The resulting model, when run with the runspin.sh script, generates 964 states and passes without errors, which is reassuring. That said, although safety is critically important, it is also quite important to avoid indefinitely stalling grace periods. The next section therefore covers validating liveness.

Validating Liveness

Although liveness can be difficult to prove, there is a simple trick that applies here. The first step is to make dyntick_nohz() indicate that it is done via a dyntick_nohz_done variable, as shown on line 26 of the following:

  1 proctype dyntick_nohz()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5   bit old_gp_idle;
  6 
  7   do
  8   :: i >= MAX_DYNTICK_LOOP_NOHZ -> break;
  9   :: i < MAX_DYNTICK_LOOP_NOHZ ->
 10     tmp = dynticks_progress_counter;
 11     atomic {
 12       dynticks_progress_counter = tmp + 1;
 13       old_gp_idle = (grace_period_state == GP_IDLE);
 14       assert((dynticks_progress_counter & 1) == 1);
 15     }
 16     atomic {
 17       tmp = dynticks_progress_counter;
 18       assert(!old_gp_idle || grace_period_state != GP_DONE);
 19     }
 20     atomic {
 21       dynticks_progress_counter = tmp + 1;
 22       assert((dynticks_progress_counter & 1) == 0);
 23     }
 24     i++;
 25   od;
 26   dyntick_nohz_done = 1;
 27 }

With this variable in place, we can add assertions to grace_period() to check for unnecessary blockage as follows:

  1 proctype grace_period()
  2 {
  3   byte curr;
  4   byte snap;
  5   bit shouldexit;
  6 
  7   grace_period_state = GP_IDLE;
  8   atomic {
  9     printf("MAX_DYNTICK_LOOP_NOHZ = %d\n", MAX_DYNTICK_LOOP_NOHZ);
 10     shouldexit = 0;
 11     snap = dynticks_progress_counter;
 12     grace_period_state = GP_WAITING;
 13   }
 14   do
 15   :: 1 ->
 16     atomic {
 17       assert(!shouldexit);
 18       shouldexit = dyntick_nohz_done;
 19       curr = dynticks_progress_counter;
 20       if
 21       :: (curr == snap) && ((curr & 1) == 0) ->
 22         break;
 23       :: (curr - snap) > 2 || (snap & 1) == 0 ->
 24         break;
 25       :: else -> skip;
 26       fi;
 27     }
 28   od;
 29   grace_period_state = GP_DONE;
 30   grace_period_state = GP_IDLE;
 31   atomic {
 32     shouldexit = 0;
 33     snap = dynticks_progress_counter;
 34     grace_period_state = GP_WAITING;
 35   }
 36   do
 37   :: 1 ->
 38     atomic {
 39       assert(!shouldexit);
 40       shouldexit = dyntick_nohz_done;
 41       curr = dynticks_progress_counter;
 42       if
 43       :: (curr == snap) && ((curr & 1) == 0) ->
 44         break;
 45       :: (curr != snap) ->
 46         break;
 47       :: else -> skip;
 48       fi;
 49     }
 50   od;
 51   grace_period_state = GP_DONE;
 52 }

We have added the shouldexit variable on line 5, which we initialize to zero on line 10. Line 17 asserts that shouldexit is not set, while line 18 sets shouldexit to the dyntick_nohz_done variable maintained by dyntick_nohz(). This assertion will therefore trigger if we attempt to take more than one pass through the wait-for-counter-flip-acknowledgment loop after dyntick_nohz() has completed execution. After all, if dyntick_nohz() is done, then there cannot be any more state changes to force us out of the loop, so going through twice in this state means an infinite loop, which in turn means no end to the grace period.

Lines 32, 39, and 40 operate in a similar manner for the second (memory-barrier) loop.

However, running this model results in failure, as line 23 is checking that the wrong variable is even. Upon failure, spin writes out a “trail” file, which records the sequence of states that lead to the failure. Use the spin -t -p -g -l dyntickRCU-base-sl-busted.spin command to cause spin to retrace this sequence of states, printing the statements executed and the values of variables. Note that the line numbers do not match the listing above due to the fact that spin takes both functions in a single file. However, the line numbers do match the full model.

We see that the dyntick_nohz() process completed at step 34 (search for “34:”), but that the grace_period() process nonetheless failed to exit the loop. The value of curr is 6 (see step 35) and that the value of snap is 5 (see step 17). Therefore the first condition on line 21 above does not hold because curr != snap, and the second condition on line 23 does not hold either because snap is odd and because curr is only one greater than snap.

So one of these two conditions has to be incorrect. Referring to the comment block in rcu_try_flip_waitack_needed() for the first condition:

	/*
	 * If the CPU remained in dynticks mode for the entire time
	 * and didn't take any interrupts, NMIs, SMIs, or whatever,
	 * then it cannot be in the middle of an rcu_read_lock(), so
	 * the next rcu_read_lock() it executes must use the new value
	 * of the counter.  So we can safely pretend that this CPU
	 * already acknowledged the counter.
	 */

The first condition does match this, because if curr == snap and if curr is even, then the corresponding CPU has been in dynticks-idle mode the entire time, as required. So let's look at the comment block for the second condition:

	/*
	 * If the CPU passed through or entered a dynticks idle phase with
	 * no active irq handlers, then, as above, we can safely pretend
	 * that this CPU already acknowledged the counter.
	 */

The first part of the condition is correct, because if curr and snap differ by two, there will be at least one even number in between, corresponding to having passed completely through a dynticks-idle phase. However, the second part of the condition corresponds to having started in dynticks-idle mode, not having finished in this mode. We therefore need to be testing curr rather than snap for being an even number.

The corrected C code is as follows:

  1 static inline int
  2 rcu_try_flip_waitack_needed(int cpu)
  3 {
  4   long curr;
  5   long snap;
  6 
  7   curr = per_cpu(dynticks_progress_counter, cpu);
  8   snap = per_cpu(rcu_dyntick_snapshot, cpu);
  9   smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
 10   if ((curr == snap) && ((curr & 0x1) == 0))
 11     return 0;
 12   if ((curr - snap) > 2 || (curr & 0x1) == 0)
 13     return 0;
 14   return 1;
 15 }

Making the corresponding correction in the model results in a correct validation with 661 states that passes without errors. However, it is worth noting that the first version of the liveness validation failed to catch this bug, due to a bug in the liveness validation itself. This liveness-validation bug was located by inserting an infinite loop in the grace_period() process, and noting that the liveness-validation code failed to detect this problem!

We have now successfully validated both safety and liveness conditions, but only for processes running and blocking. We also need to handle interrupts, a task taken up in the next section.

Interrupts

There are a couple of ways to model interrupts in Promela:

1. using C-preprocessor tricks to insert the interrupt handler between each and every statement of the dynticks_nohz() process, or
2. modeling the interrupt handler with a separate process.

A bit of thought indicated that the second approach would have a smaller state space, though it requires that the interrupt handler somehow run atomically with respect to the dynticks_nohz() process, but not with respect to the grace_period() process.

Fortunately, it turns out that Promela permits you to branch out of atomic statements. This trick allows us to have the interrupt handler set a flag, and recode dynticks_nohz() to atomically check this flag and execute only when the flag is not set. This can be accomplished with a C-preprocessor macro that takes a label and a Promela statement as follows:

  1 #define EXECUTE_MAINLINE(label, stmt) \
  2 label: skip; \
  3     atomic { \
  4       if \
  5       :: in_dyntick_irq -> goto label; \
  6       :: else -> stmt; \
  7       fi; \
  8     } \

One might use this macro as follows:

    EXECUTE_MAINLINE(stmt1, tmp = dynticks_progress_counter)

Line 2 of the macro creates the specified statement label. Lines 3-8 are an atomic block that tests the in_dyntick_irq variable, and if this variable is set (indicating that the interrupt handler is active), branches out of the atomic block back to the label. Otherwise, line 6 executes the specified statement. The overall effect is that mainline execution stalls any time an interrupt is active, as required.

Validating Interrupt Handlers

The first step is to convert dyntick_nohz() to EXECUTE_MAINLINE() form, as follows:

  1 proctype dyntick_nohz()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5   bit old_gp_idle;
  6 
  7   do
  8   :: i >= MAX_DYNTICK_LOOP_NOHZ -> break;
  9   :: i < MAX_DYNTICK_LOOP_NOHZ ->
 10     EXECUTE_MAINLINE(stmt1, tmp = dynticks_progress_counter)
 11     EXECUTE_MAINLINE(stmt2,
 12          dynticks_progress_counter = tmp + 1;
 13          old_gp_idle = (grace_period_state == GP_IDLE);
 14          assert((dynticks_progress_counter & 1) == 1))
 15     EXECUTE_MAINLINE(stmt3,
 16       tmp = dynticks_progress_counter;
 17       assert(!old_gp_idle || grace_period_state != GP_DONE))
 18     EXECUTE_MAINLINE(stmt4,
 19       dynticks_progress_counter = tmp + 1;
 20       assert((dynticks_progress_counter & 1) == 0))
 21     i++;
 22   od;
 23   dyntick_nohz_done = 1;
 24 }

It is important to note that when a group of statements is passed to EXECUTE_MAINLINE(), as in lines 11-14, all statements in that group execute atomically.

The next step is to write a dyntick_irq() process to model an interrupt handler:

  1 proctype dyntick_irq()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5   bit old_gp_idle;
  6 
  7   do
  8   :: i >= MAX_DYNTICK_LOOP_IRQ -> break;
  9   :: i < MAX_DYNTICK_LOOP_IRQ ->
 10     in_dyntick_irq = 1;
 11     if
 12     :: rcu_update_flag > 0 ->
 13       tmp = rcu_update_flag;
 14       rcu_update_flag = tmp + 1;
 15     :: else -> skip;
 16     fi;
 17     if
 18     :: !in_interrupt && (dynticks_progress_counter & 1) == 0 ->
 19       tmp = dynticks_progress_counter;
 20       dynticks_progress_counter = tmp + 1;
 21       tmp = rcu_update_flag;
 22       rcu_update_flag = tmp + 1;
 23     :: else -> skip;
 24     fi;
 25     tmp = in_interrupt;
 26     in_interrupt = tmp + 1;
 27     old_gp_idle = (grace_period_state == GP_IDLE);
 28     assert(!old_gp_idle || grace_period_state != GP_DONE);
 29     tmp = in_interrupt;
 30     in_interrupt = tmp - 1;
 31     if
 32     :: rcu_update_flag != 0 ->
 33       tmp = rcu_update_flag;
 34       rcu_update_flag = tmp - 1;
 35       if
 36       :: rcu_update_flag == 0 ->
 37         tmp = dynticks_progress_counter;
 38         dynticks_progress_counter = tmp + 1;
 39       :: else -> skip;
 40       fi;
 41     :: else -> skip;
 42     fi;
 43     atomic {
 44       in_dyntick_irq = 0;
 45       i++;
 46     }
 47   od;
 48   dyntick_irq_done = 1;
 49 }

The loop from line 7-47 models up to MAX_DYNTICK_LOOP_IRQ interrupts, with lines 8 and 9 forming the loop condition and line 45 incrementing the control variable. Line 10 tells dyntick_nohz() that an interrupt handler is running, and line 44 tells dyntick_nohz() that this handler has completed. Line 48 is used for liveness validation, much as is the corresponding line of dyntick_nohz().

Lines 11-24 model rcu_irq_enter(), and lines 25 and 26 model the relevant snippet of __irq_enter(). Lines 27 and 28 validate safety in much the same manner as do the corresponding lines of dynticks_nohz(). Lines 29 and 30 model the relevant snippet of __irq_exit(), and finally lines 31-42 model rcu_irq_exit().

  1 proctype grace_period()
  2 {
  3   byte curr;
  4   byte snap;
  5   bit shouldexit;
  6 
  7   grace_period_state = GP_IDLE;
  8   atomic {
  9     printf("MAX_DYNTICK_LOOP_NOHZ = %d\n", MAX_DYNTICK_LOOP_NOHZ);
 10     printf("MAX_DYNTICK_LOOP_IRQ = %d\n", MAX_DYNTICK_LOOP_IRQ);
 11     shouldexit = 0;
 12     snap = dynticks_progress_counter;
 13     grace_period_state = GP_WAITING;
 14   }
 15   do
 16   :: 1 ->
 17     atomic {
 18       assert(!shouldexit);
 19       shouldexit = dyntick_nohz_done && dyntick_irq_done;
 20       curr = dynticks_progress_counter;
 21       if
 22       :: (curr == snap) && ((curr & 1) == 0) ->
 23         break;
 24       :: (curr - snap) > 2 || (curr & 1) == 0 ->
 25         break;
 26       :: else -> skip;
 27       fi;
 28     }
 29   od;
 30   grace_period_state = GP_DONE;
 31   grace_period_state = GP_IDLE;
 32   atomic {
 33     shouldexit = 0;
 34     snap = dynticks_progress_counter;
 35     grace_period_state = GP_WAITING;
 36   }
 37   do
 38   :: 1 ->
 39     atomic {
 40       assert(!shouldexit);
 41       shouldexit = dyntick_nohz_done && dyntick_irq_done;
 42       curr = dynticks_progress_counter;
 43       if
 44       :: (curr == snap) && ((curr & 1) == 0) ->
 45         break;
 46       :: (curr != snap) ->
 47         break;
 48       :: else -> skip;
 49       fi;
 50     }
 51   od;
 52   grace_period_state = GP_DONE;
 53 }

The implementation of grace_period() is very similar to the earlier one. The only changes are the addition of line 10 to add the new interrupt-count parameter and changes to lines 19 and 41 to add the new dyntick_irq_done variable to the liveness checks.

This model results in a correct validation with roughly half a million states, passing without errors. However, this version of the model does not handle nested interrupts. This topic is taken up in the next section.

Validating Nested Interrupt Handlers

Nested interrupt handlers may be modeled by splitting the body of the loop in dyntick_irq() as follows:

  1 proctype dyntick_irq()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5   byte j = 0;
  6   bit old_gp_idle;
  7   bit outermost;
  8 
  9   do
 10   :: i >= MAX_DYNTICK_LOOP_IRQ && j >= MAX_DYNTICK_LOOP_IRQ -> break;
 11   :: i < MAX_DYNTICK_LOOP_IRQ ->
 12     atomic {
 13       outermost = (in_dyntick_irq == 0);
 14       in_dyntick_irq = 1;
 15     }
 16     if
 17     :: rcu_update_flag > 0 ->
 18       tmp = rcu_update_flag;
 19       rcu_update_flag = tmp + 1;
 20     :: else -> skip;
 21     fi;
 22     if
 23     :: !in_interrupt && (dynticks_progress_counter & 1) == 0 ->
 24       tmp = dynticks_progress_counter;
 25       dynticks_progress_counter = tmp + 1;
 26       tmp = rcu_update_flag;
 27       rcu_update_flag = tmp + 1;
 28     :: else -> skip;
 29     fi;
 30     tmp = in_interrupt;
 31     in_interrupt = tmp + 1;
 32     atomic {
 33       if
 34       :: outermost ->
 35         old_gp_idle = (grace_period_state == GP_IDLE);
 36       :: else -> skip;
 37       fi;
 38     }
 39     i++;
 40   :: j < i ->
 41     atomic {
 42       if
 43       :: j + 1 == i ->
 44         assert(!old_gp_idle ||
 45                grace_period_state != GP_DONE);
 46       :: else -> skip;
 47       fi;
 48     }
 49     tmp = in_interrupt;
 50     in_interrupt = tmp - 1;
 51     if
 52     :: rcu_update_flag != 0 ->
 53       tmp = rcu_update_flag;
 54       rcu_update_flag = tmp - 1;
 55       if
 56       :: rcu_update_flag == 0 ->
 57         tmp = dynticks_progress_counter;
 58         dynticks_progress_counter = tmp + 1;
 59       :: else -> skip;
 60       fi;
 61     :: else -> skip;
 62     fi;
 63     atomic {
 64       j++;
 65       in_dyntick_irq = (i != j);
 66     }
 67   od;
 68   dyntick_irq_done = 1;
 69 }

This is similar to the earlier dynticks_irq() process. It adds a second counter variable j on line 5, so that i counts entries to interrupt handlers and j counts exits. The outermost variable on line 7 helps determine when the grace_period_state variable needs to be sampled for the safety checks. The loop-exit check on line 10 is updated to require that the specified number of interrupt handlers are exited as well as entered, and the increment of i is moved to line 39, which is the end of the interrupt-entry model. Lines 12-15 set the outermost variable to indicate whether this is the outermost of a set of nested interrupts and to set the in_dyntick_irq variable that is used by the dyntick_nohz() process. Lines 32-37 capture the state of the grace_period_state variable, but only when in the outermost interrupt handler.

Line 40 has the do-loop conditional for interrupt-exit modeling: as long as we have exited fewer interrupts than we have entered, it is legal to exit another interrupt. Lines 41-48 check the safety criterion, but only if we are exiting from the outermost interrupt level. Finally, lines 63-66 increment the interrupt-exit count j and, if this is the outermost interrupt level, clears in_dyntick_irq.

This model results in a correct validation with a bit more than half a million states, passing without errors. However, this version of the model does not handle NMIs, which are taken up in the nest section.

Validating NMI Handlers

We take the same general approach for NMIs as we do for interrupts, keeping in mind that NMIs do not nest. This results in a dyntick_nmi() process as follows:

  1 proctype dyntick_nmi()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5   bit old_gp_idle;
  6 
  7   do
  8   :: i >= MAX_DYNTICK_LOOP_NMI -> break;
  9   :: i < MAX_DYNTICK_LOOP_NMI ->
 10     in_dyntick_nmi = 1;
 11     if
 12     :: rcu_update_flag > 0 ->
 13       tmp = rcu_update_flag;
 14       rcu_update_flag = tmp + 1;
 15     :: else -> skip;
 16     fi;
 17     if
 18     :: !in_interrupt && (dynticks_progress_counter & 1) == 0 ->
 19       tmp = dynticks_progress_counter;
 20       dynticks_progress_counter = tmp + 1;
 21       tmp = rcu_update_flag;
 22       rcu_update_flag = tmp + 1;
 23     :: else -> skip;
 24     fi;
 25     tmp = in_interrupt;
 26     in_interrupt = tmp + 1;
 27     old_gp_idle = (grace_period_state == GP_IDLE);
 28     assert(!old_gp_idle || grace_period_state != GP_DONE);
 29     tmp = in_interrupt;
 30     in_interrupt = tmp - 1;
 31     if
 32     :: rcu_update_flag != 0 ->
 33       tmp = rcu_update_flag;
 34       rcu_update_flag = tmp - 1;
 35       if
 36       :: rcu_update_flag == 0 ->
 37         tmp = dynticks_progress_counter;
 38         dynticks_progress_counter = tmp + 1;
 39       :: else -> skip;
 40       fi;
 41     :: else -> skip;
 42     fi;
 43     atomic {
 44       i++;
 45       in_dyntick_nmi = 0;
 46     }
 47   od;
 48   dyntick_nmi_done = 1;
 49 }

Of course, the fact that we have NMIs requires adjustments in the other components. For example, the EXECUTE_MAINLINE() macro now needs to pay attention to the NMI handler (in_dyntick_nmi) as well as the interrupt handler (in_dyntick_irq) by checking the dyntick_nmi_done variable as follows:

  1 #define EXECUTE_MAINLINE(label, stmt) \
  2 label: skip; \
  3     atomic { \
  4       if \
  5       :: in_dyntick_irq || in_dyntick_nmi -> goto label; \
  6       :: else -> stmt; \
  7       fi; \
  8     } \

We will also need to introduce an EXECUTE_IRQ() macro that checks in_dyntick_nmi in order to allow dyntick_irq() to exclude dyntick_nmi():

  1 #define EXECUTE_IRQ(label, stmt) \
  2 label: skip; \
  3     atomic { \
  4       if \
  5       :: in_dyntick_nmi -> goto label; \
  6       :: else -> stmt; \
  7       fi; \
  8     } \

It is further necessary to convert dyntick_irq() to EXECUTE_IRQ() as follows:

  1 proctype dyntick_irq()
  2 {
  3   byte tmp;
  4   byte i = 0;
  5   byte j = 0;
  6   bit old_gp_idle;
  7   bit outermost;
  8 
  9   do
 10   :: i >= MAX_DYNTICK_LOOP_IRQ && j >= MAX_DYNTICK_LOOP_IRQ -> break;
 11   :: i < MAX_DYNTICK_LOOP_IRQ ->
 12     atomic {
 13       outermost = (in_dyntick_irq == 0);
 14       in_dyntick_irq = 1;
 15     }
 16 stmt1: skip;
 17     atomic {
 18       if
 19       :: in_dyntick_nmi -> goto stmt1;
 20       :: !in_dyntick_nmi && rcu_update_flag ->
 21         goto stmt1_then;
 22       :: else -> goto stmt1_else;
 23       fi;
 24     }
 25 stmt1_then: skip;
 26     EXECUTE_IRQ(stmt1_1, tmp = rcu_update_flag)
 27     EXECUTE_IRQ(stmt1_2, rcu_update_flag = tmp + 1)
 28 stmt1_else: skip;
 29 stmt2: skip;  atomic {
 30       if
 31       :: in_dyntick_nmi -> goto stmt2;
 32       :: !in_dyntick_nmi &&
 33          !in_interrupt &&
 34          (dynticks_progress_counter & 1) == 0 ->
 35            goto stmt2_then;
 36       :: else -> goto stmt2_else;
 37       fi;
 38     }
 39 stmt2_then: skip;
 40     EXECUTE_IRQ(stmt2_1, tmp = dynticks_progress_counter)
 41     EXECUTE_IRQ(stmt2_2, dynticks_progress_counter = tmp + 1)
 42     EXECUTE_IRQ(stmt2_3, tmp = rcu_update_flag)
 43     EXECUTE_IRQ(stmt2_4, rcu_update_flag = tmp + 1)
 44 stmt2_else: skip;
 45     EXECUTE_IRQ(stmt3, tmp = in_interrupt)
 46     EXECUTE_IRQ(stmt4, in_interrupt = tmp + 1)
 47 stmt5: skip;
 48     atomic {
 49       if
 50       :: in_dyntick_nmi -> goto stmt4;
 51       :: !in_dyntick_nmi && outermost ->
 52         old_gp_idle = (grace_period_state == GP_IDLE);
 53       :: else -> skip;
 54       fi;
 55     }
 56     i++;
 57   :: j < i ->
 58 stmt6: skip;
 59     atomic {
 60       if
 61       :: in_dyntick_nmi -> goto stmt6;
 62       :: !in_dyntick_nmi && j + 1 == i ->
 63         assert(!old_gp_idle || grace_period_state != GP_DONE);
 64       :: else -> skip;
 65       fi;
 66     }
 67     EXECUTE_IRQ(stmt7, tmp = in_interrupt);
 68     EXECUTE_IRQ(stmt8, in_interrupt = tmp - 1);
 69 
 70 stmt9: skip;
 71     atomic {
 72       if
 73       :: in_dyntick_nmi -> goto stmt9;
 74       :: !in_dyntick_nmi && rcu_update_flag != 0 ->
 75         goto stmt9_then;
 76       :: else -> goto stmt9_else;
 77       fi;
 78     }
 79 stmt9_then: skip;
 80     EXECUTE_IRQ(stmt9_1, tmp = rcu_update_flag)
 81     EXECUTE_IRQ(stmt9_2, rcu_update_flag = tmp - 1)
 82 stmt9_3: skip;
 83     atomic {
 84       if
 85       :: in_dyntick_nmi -> goto stmt9_3;
 86       :: !in_dyntick_nmi && rcu_update_flag == 0 ->
 87         goto stmt9_3_then;
 88       :: else -> goto stmt9_3_else;
 89       fi;
 90     }
 91 stmt9_3_then: skip;
 92     EXECUTE_IRQ(stmt9_3_1, tmp = dynticks_progress_counter)
 93     EXECUTE_IRQ(stmt9_3_2, dynticks_progress_counter = tmp + 1)
 94 stmt9_3_else:
 95 stmt9_else: skip;
 96     atomic {
 97       j++;
 98       in_dyntick_irq = (i != j);
 99     }
100   od;
101   dyntick_irq_done = 1;
102 }

Note that we have open-coded the “if” statements (for example, lines 16-28). In addition, statements that process strictly local state (such as line 56) need not exclude dyntick_nmi().

Finally, grace_period() requires only a few changes:

  1 proctype grace_period()
  2 {
  3   byte curr;
  4   byte snap;
  5   bit shouldexit;
  6 
  7   grace_period_state = GP_IDLE;
  8   atomic {
  9     printf("MAX_DYNTICK_LOOP_NOHZ = %d\n", MAX_DYNTICK_LOOP_NOHZ);
 10     printf("MAX_DYNTICK_LOOP_IRQ = %d\n", MAX_DYNTICK_LOOP_IRQ);
 11     printf("MAX_DYNTICK_LOOP_NMI = %d\n", MAX_DYNTICK_LOOP_NMI);
 12     shouldexit = 0;
 13     snap = dynticks_progress_counter;
 14     grace_period_state = GP_WAITING;
 15   }
 16   do
 17   :: 1 ->
 18     atomic {
 19       assert(!shouldexit);
 20       shouldexit = dyntick_nohz_done &&
 21              dyntick_irq_done &&
 22              dyntick_nmi_done;
 23       curr = dynticks_progress_counter;
 24       if
 25       :: (curr == snap) && ((curr & 1) == 0) ->
 26         break;
 27       :: (curr - snap) > 2 || (curr & 1) == 0 ->
 28         break;
 29       :: else -> skip;
 30       fi;
 31     }
 32   od;
 33   grace_period_state = GP_DONE;
 34   grace_period_state = GP_IDLE;
 35   atomic {
 36     shouldexit = 0;
 37     snap = dynticks_progress_counter;
 38     grace_period_state = GP_WAITING;
 39   }
 40   do
 41   :: 1 ->
 42     atomic {
 43       assert(!shouldexit);
 44       shouldexit = dyntick_nohz_done &&
 45              dyntick_irq_done &&
 46              dyntick_nmi_done;
 47       curr = dynticks_progress_counter;
 48       if
 49       :: (curr == snap) && ((curr & 1) == 0) ->
 50         break;
 51       :: (curr != snap) ->
 52         break;
 53       :: else -> skip;
 54       fi;
 55     }
 56   od;
 57   grace_period_state = GP_DONE;
 58 }

We have added the printf() for the new MAX_DYNTICK_LOOP_NMI parameter on line 11 and added dyntick_nmi_done to the shouldexit assignments on lines 22 and 46.

The model results in a correct validation with several hundred million states, passing without errors.

Conclusions

This effort provided some lessons (re)learned:

Promela and spin can validate interrupt/NMI-handler interactions.

Documenting code can help locate bugs. In this case, the documentation effort located this bug.

Validate your code early, often, and up to the point of destruction. This effort located one subtle bug that might have been quite difficult to test or debug.

Always validate your validation code. The usual way to do this is to insert a deliberate bug and verify that the validation code catches it. Of course, if the validation code fails to catch this bug, you may also need to verify the bug itself, and so on, recursing infinitely. However, if you find yourself in this position, getting a good night's sleep can be an extremely effective debugging technique.

Finally, if cmpxchg instructions ever become inexpensive enough to tolerate them in the interrupt fastpath, their use could greatly simplify this code. The Promela model for an atomic-instruction-based implementation of this code has more than an order of magnitude fewer states, and the C code is much easier to understand. On the other hand, one must take care when using cmpxchg instructions, as some code sequences, if highly contended, can result in starvation. This situation is particularly likely to occur when part of the algorithm uses cmpxchg, other parts of the algorithm use atomic instructions that cannot fail (e.g., atomic increment), contention for the variable in question is high, and the code is running on a NUMA or a shared-cache machine. Sadly, almost all multi-socket systems with either multi-core or multi-threaded CPUs fit this description.

Acknowledgments

We are indebted to Andrew Theurer, who maintains the large-memory machine that ran the full test. We all owe a debt of gratitude to Vara Prasad for his help in rendering this article human-readable.