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

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Message-ID: <1674.1560435952@warthog.procyon.org.uk>
Date:   Thu, 13 Jun 2019 15:25:52 +0100
From:   David Howells <dhowells@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     dhowells@...hat.com, stern@...land.harvard.edu, akiyks@...il.com,
        andrea.parri@...rulasolutions.com, boqun.feng@...il.com,
        dlustig@...dia.com, j.alglave@....ac.uk, luc.maranget@...ia.fr,
        npiggin@...il.com, paulmck@...ux.ibm.com, will.deacon@....com,
        paul.burton@...s.com, 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.

Peter Zijlstra <peterz@...radead.org> wrote:

> 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:

To quote memory-barriers.txt:

 (*) smp_mb__before_atomic();
 (*) smp_mb__after_atomic();

     These are for use with atomic (such as add, subtract, increment and
     decrement) functions that don't return a value, especially when used for
     reference counting.  These functions do not imply memory barriers.

so it's entirely to be expected?

David