|| ||Thomas Gleixner <tglx-AT-linutronix.de> |
|| ||Ingo Molnar <mingo-AT-elte.hu> |
|| ||Re: [rfc] "fair" rw spinlocks |
|| ||Mon, 30 Nov 2009 22:12:45 +0100 (CET)|
|| ||Linus Torvalds <torvalds-AT-linux-foundation.org>,
Peter Zijlstra <a.p.zijlstra-AT-chello.nl>,
Christoph Hellwig <hch-AT-infradead.org>,
Nick Piggin <npiggin-AT-suse.de>,
Linux Kernel Mailing List <linux-kernel-AT-vger.kernel.org>|
|| ||Article, Thread
On Mon, 30 Nov 2009, Ingo Molnar wrote:
> * Linus Torvalds <firstname.lastname@example.org> wrote:
> > On Mon, 30 Nov 2009, Christoph Hellwig wrote:
> > >
> > > How long will this use be around? I've seen some slow progress toward
> > > replacing most read side uses of the task list lock with RCU. While we
> > > still have lots of read side users now I wonder when they'll go away.
> > tasklist_lock is pretty nasty. I threw out "replace it with RCU"
> > because it would be nice, but the data structures used are not just
> > simple linked lists that we have RCU helpers for traversing.
> > There are various real exclusion rules about things like
> > 'tsk->exit_state' etc, which do not translate directly to RCU usage.
> > Of course, _maybe_ all the places that care already take the thing for
> > writing and would just automatically have exclusion anyway.
> > So I'd love to see somebody try to do the conversion. To a first
> > approximation, you probably could do
> > - turn tasklist_lock into a spinlock
> > - sed 's/write_lock_irq(&tasklist_lock)/spin_lock(&tasklist_lock)/g'
> > sed 's/write_unlock_irq(&tasklist_lock)/spin_unlock(&tasklist_lock)/g'
> > - sed 's/read_lock(&tasklist_lock)/rcu_read_lock()/g'
> > sed 's/read_unlock(&tasklist_lock)/rcu_read_unlock()/g'
> > - make all the task lists use the RCU versions of the list routines
> > - free the task structure using RCU
> > and you'd be _pretty_ close to a working system.
> In -rt we've got that in essence, and it's indeed working fine (with a
> few caveats). A few RCU conversions of tasklist_lock usage in that area
> even trickled upstream, because the simple lock would hurt so much under
I think the conversion Linus proposed is pretty feasible. I went
through the read_lock sites and most of them are protecting function
calls which we already use under rcu_read_lock() in other places like
find_task* and thread or pid iterators.
There are a few non obvious ones in signal.c and posix-cpu-timers.c
(what a surprise) but nothing looks too scary.
If nobody beats me I'm going to let sed loose on the kernel, lift the
task_struct rcu free code from -rt and figure out what explodes.
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