linux-kernel - Re: linux-next: build warnings after merge of the access

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Date:	Thu, 26 Mar 2015 15:51:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: linux-next: build warnings after merge of the access_once tree

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.


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