From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([208.118.235.92]:52030)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1TC4eE-0004rf-Q6
	for qemu-devel@nongnu.org; Thu, 13 Sep 2012 04:19:43 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1TC4eD-0000Pi-Pq
	for qemu-devel@nongnu.org; Thu, 13 Sep 2012 04:19:42 -0400
Received: from mx1.redhat.com ([209.132.183.28]:21570)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1TC4eD-0000Pa-HM
	for qemu-devel@nongnu.org; Thu, 13 Sep 2012 04:19:41 -0400
Message-ID: <50519716.5030402@redhat.com>
Date: Thu, 13 Sep 2012 10:19:34 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1347349912-15611-1-git-send-email-qemulist@gmail.com>
	<1347349912-15611-2-git-send-email-qemulist@gmail.com>
	<504EF077.9080107@redhat.com>
	<CAJnKYQmH3DL8mt9znRqbA81OvorB-3FUSmV=ETOAt+vekMiZQA@mail.gmail.com>
	<505195CF.8040002@redhat.com>
In-Reply-To: <505195CF.8040002@redhat.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH V3 01/11] atomic: introduce atomic
	operations
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Avi Kivity <avi@redhat.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, liu ping fan <qemulist@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>, qemu-devel@nongnu.org

Il 13/09/2012 10:14, Avi Kivity ha scritto:
>>>> >>> +static inline void atomic_set(Atomic *v, int i)
>>>> >>> +{
>>>> >>> +    v->counter = i;
>>>> >>> +}
>>>> >>> +
>>>> >>> +static inline int atomic_read(Atomic *v)
>>>> >>> +{
>>>> >>> +    return v->counter;
>>>> >>> +}
>>>> >>>
>>> >>
>>> >> So these two operations don't get mangled by the optimizer.
>>> >>
>> > Browsing linux code and reading lkml, find some similar material. But
>> > they have moved volatile from ->counter to function - atomic_read().
>> > As to atomic_read(), I think it need to prevent optimizer from
>> > refetching issue, but as to atomic_set(), do we need ?
> I think so, to prevent reordering.

Agreed.  Alternatively, stick a barrier() before and after the
assignment and read.

But I don't really see the point in wrapping atomically-accessed
variables in a struct.  Are we going to add a variant for long, a
variant for pointers, etc.?

I already proposed my version of this at
http://github.com/bonzini/qemu/commit/1b439343.

Paolo