futex: priority wakeup miss ordering

futex: priority wakeup miss ordering

[Daniel Walker]

from commit ec92d08292d3e9b0823eba138a4564d2d39f25c7

The wakeup ordering should be in priority order. However if the priority
of a task is modified while it's waiting on the futex you will get an
out of order wakeup.

If you use a priority inheriting mutex this is correct. It's because the
rtmutex hooks into sched_setscheduler to catch any priority changes,
then re-queues with the new priority.. But the futex doesn't have this
hook.

Was this intentionally left out of this priority wake up patch? I
thought I would let you know in case you had a specific reason for not
handling this.. 

Daniel

Re: futex: priority wakeup miss ordering

[Andrew Morton]

On Wed, 07 May 2008 14:22:58 -0700 Daniel Walker <dwalker@mvista.com> wrote:

> 
> 
> from commit ec92d08292d3e9b0823eba138a4564d2d39f25c7
> 
> The wakeup ordering should be in priority order. However if the priority
> of a task is modified while it's waiting on the futex you will get an
> out of order wakeup.
> 
> If you use a priority inheriting mutex this is correct. It's because the
> rtmutex hooks into sched_setscheduler to catch any priority changes,
> then re-queues with the new priority.. But the futex doesn't have this
> hook.
> 
> Was this intentionally left out of this priority wake up patch? I
> thought I would let you know in case you had a specific reason for not
> handling this.. 
> 

Pierre is no longer @bull.net.

Re: futex: priority wakeup miss ordering

[Thomas Gleixner]

On Wed, 7 May 2008, Daniel Walker wrote:
> 
> from commit ec92d08292d3e9b0823eba138a4564d2d39f25c7
> 
> The wakeup ordering should be in priority order. However if the priority
> of a task is modified while it's waiting on the futex you will get an
> out of order wakeup.
> 
> If you use a priority inheriting mutex this is correct. It's because the
> rtmutex hooks into sched_setscheduler to catch any priority changes,
> then re-queues with the new priority.. But the futex doesn't have this
> hook.
> 
> Was this intentionally left out of this priority wake up patch? I
> thought I would let you know in case you had a specific reason for not
> handling this.. 

IIRC this has been discussed before. The priority ordering of the
plain mutex waiters is an optimization without guarantees of
correctness across a priority change. It's simply not worth the
additional complexity and overhead for that corner case.

Thanks,

	tglx

Re: futex: priority wakeup miss ordering

[Daniel Walker]

On Thu, 2008-05-08 at 12:06 +0200, Thomas Gleixner wrote:

> IIRC this has been discussed before. The priority ordering of the
> plain mutex waiters is an optimization without guarantees of
> correctness across a priority change. It's simply not worth the
> additional complexity and overhead for that corner case.

I've gotten a number of complains about this .. I think it's assumed
that this feature was implementing posix priority queuing for the
standard futex, but it can't be if there are any miss ordered wakeups.

Daniel