Re: robust futex heap support patch

From: Randy.Dunlap
Date: Tue Dec 06 2005 - 20:22:38 EST


On Tue, 6 Dec 2005 15:06:12 +0100 (MET) Esben Nielsen wrote:

> On Mon, 5 Dec 2005, david singleton wrote:
>
> >
> > On Dec 5, 2005, at 2:30 PM, Esben Nielsen wrote:
> >
> > I'm currently trying to close a race condition between futex_wait_robust
> > and futex_wake_robust that Dave Carlson is seeing on his SMP system.
> >
> > The scenario is as follows:
> >
> > Thread A locks an pthread_mutex via the fast path and does not enter
> > the kernel.
> >
> > Thread B tries to lock the lock and sees it is already locked. Thread
> > B sets the
> > waiters flag in the lock and enters the kernel to lock the lock on
> > behalf of
> > thread A and then block on the mutex waiting for it's release.
> >
> > Thread A unlocks the lock and sees the waiters flag set. Thread A gets
> > to the futex_wake_robust before Thread B can get to futex_wait_robust.
> >
> > Thread A sees that it does not own the lock in the kernel and was
> > returning EINVAL.
> >
> > patch-2.6.14-rt21-rf8 was a preliminary patch for Dave Carlson to try
> > and get more information about
> > the race condition. ( rf8 and rf9 are still returning EAGAIN from
> > thread B trying to
> > do the futex_wait_robust and the library should be retrying with
> > EAGAIN, but it currently isn't).
> >
>
> *nod*
> I just pointed out that you can't make thread A loop the way you do.
>
> What I would do was to do the user space flag checks while having the
> raw spinlock of the rt_mutex. That way you are sure that stuff in the
> kernel is serialized. But I don't know what to do exactly to do in
> there....
>
>
> > When I get the race condition closed I'll post the patch and notify
> > everyone on lkml
> > and the robustmutexes mailing lists.
>
> Where can I sign up to that mailing list? I would like to follow the
> development, although I don't have much time to contibute.

http://lists.osdl.org/mailman/listinfo/robustmutexes/


---
~Randy
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