linux-kernel - Re: [PATCH v2 0/4] atomic: Fixes to smp_mb__{before,after}

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Message-ID: <20190831090055.GH2369@hirez.programming.kicks-ass.net>
Date:   Sat, 31 Aug 2019 11:00:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     stern@...land.harvard.edu, akiyks@...il.com,
        andrea.parri@...rulasolutions.com, boqun.feng@...il.com,
        dlustig@...dia.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, npiggin@...il.com, paulmck@...ux.ibm.com,
        will.deacon@....com, paul.burton@...s.com
Cc:     linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH v2 0/4] atomic: Fixes to smp_mb__{before,after}_atomic()
 and mips.

On Thu, Jun 13, 2019 at 03:43:17PM +0200, Peter Zijlstra wrote:
> Hi,
> 
> This all started when Andrea Parri found a 'surprising' behaviour for x86:
> 
>   http://lkml.kernel.org/r/20190418125412.GA10817@andrea
> 
> Basically we fail for:
> 
> 	*x = 1;
> 	atomic_inc(u);
> 	smp_mb__after_atomic();
> 	r0 = *y;
> 
> Because, while the atomic_inc() implies memory order, it
> (surprisingly) does not provide a compiler barrier. This then allows
> the compiler to re-order like so:
> 
> 	atomic_inc(u);
> 	*x = 1;
> 	smp_mb__after_atomic();
> 	r0 = *y;
> 
> Which the CPU is then allowed to re-order (under TSO rules) like:
> 
> 	atomic_inc(u);
> 	r0 = *y;
> 	*x = 1;
> 
> And this very much was not intended.
> 
> This had me audit all the (strong) architectures that had weak
> smp_mb__{before,after}_atomic: ia64, mips, sparc, s390, x86, xtensa.
> 
> Of those, only x86 and mips were affected. Looking at MIPS to solve this, led
> to the other MIPS patches.
> 
> All these patches have been through 0day for quite a while.
> 
> Paul, how do you want to route the MIPS bits?

Paul; afaict the MIPS patches still apply (ie. they've not made their
way into Linus' tree yet).

I thought you were going to take them?