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Date: Thu, 1 Feb 2018 16:52:42 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Stafford Horne <shorne@gmail.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Jonas Bonn <jonas@southpole.se>,
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        David Howells <dhowells@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: asm-generic: Disallow no-op mb() for SMP systems
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On Thu, Feb 01, 2018 at 04:50:07PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 01, 2018 at 03:39:51PM +0000, Will Deacon wrote:
> > > I could've gotten my brain in a twist or course, which isn't _that_
> > > unusual. I never seem to be able to quite remember the holes you have
> > > with ll/sc on arm64 :-)
> > 
> > Is smp_mb__before_atomic supposed to provide ordering guarantees if it's
> > used before a failed cmpxchg? If so, I think it's needed here because the
> > l.swa might not even execute. Or did I just invent another problem?
> 
> I think it should do so indeed (and afaik all our current archs are good
> that way).

See commit:

  34d54f3d6917 ("locking/pvqspinlock: Relax cmpxchg's to improve performance on some architectures")