Re: robust futex heap support patch

[Esben Nielsen]

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.

Esben

> 
> David
> 
> 
> 
> > Hi,
> >  I got a little time to look at your current patch (2.6.14-rt21-rf8).
> > I noticed a problem in futex_wake_robust(). You have a "goto retry" to
> > solve the following situation:
> >
> >   Task A                             Task B
> >       takes futex in userspace
> >                                      Tries to take mutex and sets the
> >                                      waiting bit in user space
> >       releases futex, notices task B
> >       calls kernel and enters
> >       futex_wake_robust()
> >
> >     retry:
> >       if not owner in rt_mutex
> >          goto retry;
> >                                      Calls kernel
> >                                      Makes A owner in rt_mutex
> >                                      blocks
> >        Leaves retry-loop and
> >        completes the futex wake
> >        operation as normally.
> >
> >
> > However,  if Task A is RT on a UP machine it will go on in the retry 
> > loop
> > forever. Task B will never get the CPU to complete it's kernel-call.
> >
> > You have probably by manipulating the userspace flag from within the
> > rt_mutex code :-(
> >
> > Esben
> >
> >
> > On Fri, 25 Nov 2005, david singleton wrote:
> >
> >> There is a new patch, patch-2.6.14-rt15-rf1,  that adds support for
> >> robust and priority inheriting
> >> pthread_mutexes on the 'heap'.
> >>
> >> The previous patches only supported either file based pthread_mutexes
> >> or mmapped anonymous memory based pthread_mutexes.  This patch allows
> >> pthread_mutexes
> >> to be 'malloc'ed while using the PTHREAD_MUTEX_ROBUST_NP attribute
> >> or PTHREAD_PRIO_INHERIT attribute.
> >>
> >> The patch can be found at:
> >>
> >> http://source.mvista.com/~dsingleton
> >>
> >> David
> >>
> >> -
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> >
> 

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