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

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Date:   Thu, 13 Jun 2019 12:58:11 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     David Howells <dhowells@...hat.com>
cc:     Peter Zijlstra <peterz@...radead.org>, <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.

On Thu, 13 Jun 2019, David Howells wrote:

> 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?

The text is perhaps ambiguous.  It means that the atomic functions
which don't return values -- like atomic_inc() -- do not imply memory
barriers.  It doesn't mean that smp_mb__before_atomic() and
smp_mb__after_atomic() do not imply memory barriers.

The behavior Peter described is not to be expected.  The expectation is 
that the smp_mb__after_atomic() in the example should force the "*x = 
1" store to execute before the "r0 = *y" load.  But on current x86 it 
doesn't force this, for the reason explained in the description.

Alan