linux-kernel - Re: asm-generic: Disallow no-op mb() for SMP systems

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Message-ID: <20180201155242.GL2295@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Feb 2018 16:52:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Stafford Horne <shorne@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        David Howells <dhowells@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: asm-generic: Disallow no-op mb() for SMP systems

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")