Re: robust futex heap support patch

[david singleton <dsingleton@mvista.com>]

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).

When I get the race condition closed I'll post the patch and notify 
everyone on lkml
and the robustmutexes mailing lists.

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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