From: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
To: Mikulas Patocka <mpatocka-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: James Hogan <james.hogan-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>,
linux-metag-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: Re: [PATCH] metag: fix memory barriers
Date: Fri, 9 May 2014 11:11:31 +0200 [thread overview]
Message-ID: <20140509091131.GL30445@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <alpine.LRH.2.02.1405081546110.4346-Hpncn10jQN4oNljnaZt3ZvA+iT7yCHsGwRM8/txMwJMAicBL8TP8PQ@public.gmane.org>
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On Thu, May 08, 2014 at 03:51:37PM -0400, Mikulas Patocka wrote:
> Volatile access doesn't really imply the compiler barrier. Volatile access
> is only ordered with respect to other volatile accesses, it isn't ordered
> with respect to general memory accesses. Gcc may reorder memory accesses
> around volatile access, as we can see in this simple example (if we
> compile it with optimization, both increments of *b will be collapsed to
> just one):
>
> void fn(volatile int *a, long *b)
> {
> (*b)++;
> *a = 10;
> (*b)++;
> }
>
> Consequently, we need the compiler barrier after a write to the volatile
> variable, to make sure that the compiler doesn't reorder the volatile
> write with something else.
>
> Signed-off-by: Mikulas Patocka <mpatocka-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Acked-by: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
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WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: James Hogan <james.hogan@imgtec.com>,
linux-metag@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] metag: fix memory barriers
Date: Fri, 9 May 2014 11:11:31 +0200 [thread overview]
Message-ID: <20140509091131.GL30445@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <alpine.LRH.2.02.1405081546110.4346@file01.intranet.prod.int.rdu2.redhat.com>
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On Thu, May 08, 2014 at 03:51:37PM -0400, Mikulas Patocka wrote:
> Volatile access doesn't really imply the compiler barrier. Volatile access
> is only ordered with respect to other volatile accesses, it isn't ordered
> with respect to general memory accesses. Gcc may reorder memory accesses
> around volatile access, as we can see in this simple example (if we
> compile it with optimization, both increments of *b will be collapsed to
> just one):
>
> void fn(volatile int *a, long *b)
> {
> (*b)++;
> *a = 10;
> (*b)++;
> }
>
> Consequently, we need the compiler barrier after a write to the volatile
> variable, to make sure that the compiler doesn't reorder the volatile
> write with something else.
>
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Cc: stable@vger.kernel.org
Acked-by: Peter Zijlstra <peterz@infradead.org>
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next prev parent reply other threads:[~2014-05-09 9:11 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-08 19:51 [PATCH] metag: fix memory barriers Mikulas Patocka
2014-05-08 19:51 ` Mikulas Patocka
[not found] ` <alpine.LRH.2.02.1405081546110.4346-Hpncn10jQN4oNljnaZt3ZvA+iT7yCHsGwRM8/txMwJMAicBL8TP8PQ@public.gmane.org>
2014-05-09 9:11 ` Peter Zijlstra [this message]
2014-05-09 9:11 ` Peter Zijlstra
2014-05-09 13:58 ` James Hogan
2014-05-09 13:58 ` James Hogan
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