Messages in this thread | | | Date | Wed, 30 Oct 2013 21:56:29 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read_unlock |
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On Thu, Oct 31, 2013 at 01:26:05AM +0200, Michael S. Tsirkin wrote: > > > Paul, could you review this patch please? > > > Documentation/memory-barriers.txt says that unlock has a weaker > > > uni-directional barrier, but in practice srcu_read_unlock calls > > > smp_mb(). > > > > > > Is it OK to rely on this? If not, can I add > > > smp_mb__after_srcu_read_unlock (making it an empty macro for now) > > > so we can avoid an actual extra smp_mb()? > > > > Please use smp_mb__after_srcu_read_unlock(). After all, it was not > > that long ago that srcu_read_unlock() contained no memory barriers, > > and perhaps some day it won't need to once again. > > > > Thanx, Paul > > > > Thanks! > Something like this will be enough? > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h > index c114614..9b058ee 100644 > --- a/include/linux/srcu.h > +++ b/include/linux/srcu.h > @@ -237,4 +237,18 @@ static inline void srcu_read_unlock(struct srcu_struct *sp, int idx) > __srcu_read_unlock(sp, idx); > } > > +/** > + * smp_mb__after_srcu_read_unlock - ensure full ordering after srcu_read_unlock > + * > + * Converts the preceding srcu_read_unlock into a two-way memory barrier. > + * > + * Call this after srcu_read_unlock, to guarantee that all memory operations > + * that occur after smp_mb__after_srcu_read_unlock will appear to happen after > + * the preceding srcu_read_unlock. > + */ > +static inline void smp_mb__after_srcu_read_unlock(void) > +{ > + /* __srcu_read_unlock has smp_mb() internally so nothing to do here. */ > +} > + > #endif
Yep, that should do it!
Thanx, Paul
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