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

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Date:	Fri, 22 Apr 2016 17:53:11 +0100
From:	Will Deacon <will.deacon@....com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, waiman.long@....com,
	mingo@...hat.com, paulmck@...ux.vnet.ibm.com,
	torvalds@...ux-foundation.org, dave@...olabs.net
Subject: Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait()

On Sat, Apr 23, 2016 at 12:08:57AM +0800, Boqun Feng wrote:
> 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.

Maybe, but that makes it difficult for me to use a load-acquire instruction
for the ACQUIRE case.

Will