Re: Patch for lost wakeups

From: Oleg Nesterov
Date: Sun Aug 11 2013 - 13:32:57 EST

On 08/11, Oleg Nesterov wrote:
> On 08/09, Linus Torvalds wrote:
> >
> > I guess that instead of a "smp_wmb()", we could do another
> > "smp_mb__before_spinlock()" thing, like we already allow for other
> > architectures to do a weaker form of mb in case the spinlock is
> > already a full mb. That would allow avoiding extra synchronization. Do
> > a
> >
> > #ifndef smp_mb__before_spinlock
> > #define smp_mb__before_spinlock() smp_wmb()
> > #endif
> >
> > in <linux/spinlock.h> to not force everybody to implement it. Because
> > a wmb+acquire should be close enough to a full mb that nobody cares
> > (ok, so reads could move into the critical region from outside, but by
> > the time anybody has called "schedule()", I can't see it mattering, so
> > "close enough").
> Yes, this is what I tried to suggest. And of course we should turn that
> wmb() in try_to_wake_up() into smp_mb__before_spinlock().
> I event started the patch, but we already have smp_mb__after_lock(), so
> it should be smp_mb__before_lock() for consistency and we need to turn
> it to "define" too. Or change ARCH_HAS_SMP_MB_AFTER_LOCK, or add

Ah, please ignore. According to /bin/grep smp_mb__before_lock() no longer
have users. So we can probably simply kill it in the same patch. I'll try
to write the changelog and send it tomorrow.


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