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

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Message-ID: <20150326150845.GG2805@arm.com>
Date:	Thu, 26 Mar 2015 15:08:45 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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 02:51:44PM +0000, Peter Zijlstra wrote:
> 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.

In which case, on the premise that we comment the barrier():

  Acked-by: Will Deacon <will.deacon@....com>

As an aside, ARMv7 (32-bit) with LPAE *can* emit single-copy atomic 64-bit
memory accesses and we rely on that for things like atomic64_read and
writing ptes. If we see WRITE_ONCE(pte), then we'll have genuine issues
with the way it's currently implemented.

Will
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