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Date: Thu, 26 Mar 2015 15:51:44 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
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        "linux-next@vger.kernel.org" <linux-next@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>
Subject: Re: linux-next: build warnings after merge of the access_once tree
Message-ID: <20150326145144.GZ21418@twins.programming.kicks-ass.net>
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On Thu, Mar 26, 2015 at 02:41:54PM +0000, Will Deacon wrote:
> > +++ b/lib/lockref.c
> > @@ -18,7 +18,8 @@
> >  #define CMPXCHG_LOOP(CODE, SUCCESS) do {					\
> >  	struct lockref old;							\
> >  	BUILD_BUG_ON(sizeof(old) != 8);						\
> > -	old.lock_count = READ_ONCE(lockref->lock_count);			\
> > +	barrier();								\
> > +	old.lock_count = lockref->lock_count;					\
> >  	while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) {  	\
> >  		struct lockref new = old, prev = old;				\
> >  		CODE								\
> 
> Is ACCESS_ONCE actually going away? 

I've been arguing for that yes, having two APIs for the 'same' thing is
confusing at best, and as the comment near the READ_ONCE() thing
explains, ACCESS_ONCE() has serious, silent, issues.

> It has its problems, but I think it's
> what we want here and reads better than magic barrier() imo.

Yeah, but its also misleading because we rely on silent fail. Part of
the ACCESS_ONCE() semantics is that it should avoid split loads, and
we're here actually relying on emitting just that.