From: David Howells <dhowells@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: dhowells@redhat.com, stern@rowland.harvard.edu, akiyks@gmail.com,
andrea.parri@amarulasolutions.com, boqun.feng@gmail.com,
dlustig@nvidia.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
npiggin@gmail.com, paulmck@linux.ibm.com, will.deacon@arm.com,
paul.burton@mips.com, linux-kernel@vger.kernel.org,
torvalds@linux-foundation.org
Subject: Re: [PATCH v2 0/4] atomic: Fixes to smp_mb__{before,after}_atomic() and mips.
Date: Thu, 13 Jun 2019 15:25:52 +0100 [thread overview]
Message-ID: <1674.1560435952@warthog.procyon.org.uk> (raw)
In-Reply-To: <20190613134317.734881240@infradead.org>
Peter Zijlstra <peterz@infradead.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
next prev parent reply other threads:[~2019-06-13 15:02 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-13 13:43 [PATCH v2 0/4] atomic: Fixes to smp_mb__{before,after}_atomic() and mips Peter Zijlstra
2019-06-13 13:43 ` [PATCH v2 1/4] mips/atomic: Fix cmpxchg64 barriers Peter Zijlstra
2019-06-13 13:43 ` [PATCH v2 2/4] mips/atomic: Fix loongson_llsc_mb() wreckage Peter Zijlstra
2019-06-13 13:43 ` [PATCH v2 3/4] mips/atomic: Fix smp_mb__{before,after}_atomic() Peter Zijlstra
2019-06-13 13:43 ` [PATCH v2 4/4] x86/atomic: " Peter Zijlstra
2019-06-13 14:02 ` Will Deacon
2019-06-13 14:25 ` David Howells [this message]
2019-06-13 16:58 ` [PATCH v2 0/4] atomic: Fixes to smp_mb__{before,after}_atomic() and mips Alan Stern
2019-06-13 18:00 ` Peter Zijlstra
2019-06-13 16:32 ` Paul Burton
2019-06-13 17:48 ` Peter Zijlstra
2019-08-31 9:00 ` Peter Zijlstra
2019-08-31 10:02 ` Paul Burton
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