Re: [PATCH -v11][RFC] mutex: implement adaptive spinning

Peter Zijlstra
2009-01-14T17:22:36Z

On Wed, 2009-01-14 at 18:18 +0100, Nick Piggin wrote:

> > @@ -173,21 +237,21 @@ __mutex_lock_common(struct mutex *lock,
> > spin_unlock_mutex(&lock->wait_lock, flags);
> >
> > debug_mutex_free_waiter(&waiter);
> > + preempt_enable();
> > return -EINTR;
> > }
> > __set_task_state(task, state);
> >
> > /* didnt get the lock, go to sleep: */
> > spin_unlock_mutex(&lock->wait_lock, flags);
> > - schedule();
> > + __schedule();
>
> Why does this need to do a preempt-disabled schedule? After we schedule
> away, the next task can do arbitrary things or reschedule itself, so if
> we have not anticipated such a condition here, then I can't see what
> __schedule protects. At least a comment is in order?

From:
http://programming.kicks-ass.net/kernel-patches/mutex-adaptive-spin/mutex-preempt.patch

Subject: mutex: preemption fixes
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Wed Jan 14 15:36:26 CET 2009

The problem is that dropping the spinlock right before schedule is a voluntary
preemption point and can cause a schedule, right after which we schedule again.

Fix this inefficiency by keeping preemption disabled until we schedule, do this
by explicitly disabling preemption and providing a schedule() variant that
assumes preemption is already disabled.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

> Pity to add the call overhead to schedule just for this case.

Good point, seeing any way around that?

> BTW. __schedule shouldn't need to be asmlinkage?

TBH I've no clue, probably not, Ingo?