A new path to the refrigerator [LWN.net]

One of the trickier parts of the software suspend subsystem is the "refrigerator," the code which puts all processes on hold so that the system can be suspended in a quiet state. Last week, this page looked at some issues which come up in choosing which processes to freeze and when to freeze them. Another area of work, however, is the mechanism by which the freezing actually happens.

The in-kernel software suspend code puts processes on hold with the following steps:

The process flags (stored in the flags field of the task_struct structure) gets the PF_FREEZE bit set.
A signal is delivered to the process, causing it to execute briefly.
Eventually the process notices the PF_FREEZE flag and calls refrigerator(). That call replaces PF_FREEZE with PF_FROZEN and puts the process into an unrunnable state (TASK_UNINTERRUPTIBLE).

This mechanism does work, but it has a couple of problems. The PF_* flags require some support in the scheduler, which would be nice to avoid. The real issue, though, is that accessing another process's flags requires locking to avoid race conditions. Adding that sort of locking to the software suspend code, however, is hard to do without risking deadlocks. So the suspend code simply sets the PF_FREEZE flag without locking and hopes for the best; this is one of the reasons why software suspend has never really been supported on SMP systems.

Christoph Lameter has posted a set of patches aimed at fixing these issues. With his patch, the PF_FREEZE and PF_FROZEN flags go away. Instead, struct task_struct gets a new field called todo. This field is a notifier_block pointer; whenever any part of the kernel wants a particular process to run a function in its own context, the kernel can put a notifier request onto todo. At various places in the kernel, the todo list is checked, and any notifier requests which have been put there are executed.

With this mechanism, there is no need for any special process flags. The suspend code simply adds a todo item for each process asking it to freeze itself. It is still necessary to deliver a signal to each process to force it to run in the kernel; otherwise, processes waiting on I/O (or which never call out of user space) would not execute the notifier. The actual "frozen" state is implemented with a completion in Christoph's patch, meaning that unfreezing everybody is a simple matter of a call to complete_all().

Christoph thinks that the todo mechanism may be useful beyond software suspend. A number of places in the kernel have to make changes which are best run in the context of a specific process; the code to make those changes happen can, at times, be a little ugly. The todo list is a straightforward way of running code directly in the context of interest, potentially simplifying the kernel in a few places. The patch has not made it into -mm as of this writing, but there does not appear to be any great obstacle to its inclusion there.

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