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SubjectRe: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t
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On Mon, 2010-07-12 at 07:45 -0400, Steven Rostedt wrote:
> On Sun, 2010-07-11 at 15:33 +0200, Mike Galbraith wrote:
> > On Sat, 2010-07-10 at 21:41 +0200, Mike Galbraith wrote:
>
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index a6cec32..ef489f3 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -2255,7 +2255,14 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
> > /* Queue the futex_q, drop the hb lock, wait for wakeup. */
> > futex_wait_queue_me(hb, &q, to);
> >
> > - spin_lock(&hb->lock);
> > + /*
> > + * Non-blocking synchronization point with futex_requeue().
> > + *
> > + * We dare not block here because this will alter PI state, possibly
> > + * before our waker finishes modifying same in wakeup_next_waiter().
> > + */
> > + while(!spin_trylock(&hb->lock))
> > + cpu_relax();
>
> I agree that this would work. But I wonder if this should have an:
>
> #ifdef PREEMPT_RT
> [...]
> #else
> spin_lock(&hb->lock);
> #endif
>
> around it. Or encapsulate this lock in a macro that does the same thing
> (just to keep the actual code cleaner)

Yeah, it should. I'll wait to see what Darren/others say about holding
the wakee's pi_lock across wakeup to plug it. If he submits something
along that line, I can bin this.

-Mike



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