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>,
"H. Peter Anvin" <hpa@zytor•com>,
"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
Date: Thu, 26 Mar 2015 15:51:44 +0100 [thread overview]
Message-ID: <20150326145144.GZ21418@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20150326144153.GE2805@arm.com>
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.
next prev parent reply other threads:[~2015-03-26 14:51 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-26 8:31 linux-next: build warnings after merge of the access_once tree Stephen Rothwell
2015-03-26 10:11 ` Christian Borntraeger
2015-03-26 10:34 ` Peter Zijlstra
2015-03-26 13:27 ` Will Deacon
2015-03-26 14:22 ` Peter Zijlstra
2015-03-26 14:41 ` Will Deacon
2015-03-26 14:51 ` Peter Zijlstra [this message]
2015-03-26 15:08 ` Will Deacon
2015-03-26 16:15 ` Linus Torvalds
2015-03-26 16:21 ` Linus Torvalds
2015-03-26 16:36 ` Peter Zijlstra
2015-03-26 16:44 ` Peter Zijlstra
2015-03-26 16:45 ` Peter Zijlstra
[not found] ` <CA+55aFw1WHJqSj+z-mJGY-kxrg_OsGp9jK9VBi+wB4zPgCkv_w@mail.gmail.com>
2015-03-26 17:07 ` Peter Zijlstra
2015-03-26 17:17 ` Will Deacon
2015-03-26 17:23 ` Christian Borntraeger
2015-03-26 19:42 ` Christian Borntraeger
2015-03-26 16:28 ` Peter Zijlstra
[not found] ` <CA+55aFzUPPSHakwbp-Y-SaXB+o1=V6rOknz7L3AYNXNPU1MSfg@mail.gmail.com>
2015-03-26 17:12 ` Paul E. McKenney
2015-03-26 17:24 ` Christian Borntraeger
2015-03-26 17:52 ` Linus Torvalds
2015-03-26 18:54 ` Christian Borntraeger
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