Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait()

[Boqun Feng <boqun.feng@gmail.com>]

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On Tue, Apr 12, 2016 at 05:59:41PM +0100, Will Deacon wrote:
[...]
> > +static inline void __cmpwait(volatile void *ptr, unsigned long val, int size)
> > +{
> > +	switch (size) {
> > +	case 1: return __cmpwait_case_1(ptr, val);
> > +	case 2: return __cmpwait_case_2(ptr, val);
> > +	case 4: return __cmpwait_case_4(ptr, val);
> > +	case 8: return __cmpwait_case_8(ptr, val);
> > +	default: BUILD_BUG();
> > +	}
> > +
> > +	unreachable();
> > +}
> > +
> > +#define cmpwait(ptr, val) \
> > +	__cmpwait((ptr), (unsigned long)(val), sizeof(*(ptr)))
> 
> We might want to call this cmpwait_relaxed, in case we decide to add
> fenced versions in the future. Or just make it cmpwait_acquire and
> remove the smp_rmb() from smp_cond_load_acquire(). Dunno.
> 

How about replace smp_rmb() with a smp_acquire_barrier__after_cmpwait()?
This barrier is designed to provide an ACQUIRE ordering when combining a
cmpwait() .

And cmpwait() only has minimal ordering guarantee, but if it is actually
an ACQUIRE, then the corresponding smp_acquire_barrier__after_cmpwait()
is just empty.

We might need this special barrier on ppc, because we can implement it
with "isync" given that cmpwait() has control dependency and ctrl+isync
is ACQUIRE on ppc.

Thoughts?

Regards,
Boqun

> Will

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