From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:48799)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <cota@braap.org>) id 1b2qNp-00055M-IB
	for qemu-devel@nongnu.org; Tue, 17 May 2016 21:34:48 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <cota@braap.org>) id 1b2pLe-00035L-LU
	for qemu-devel@nongnu.org; Tue, 17 May 2016 20:28:29 -0400
Received: from out5-smtp.messagingengine.com ([66.111.4.29]:33486)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <cota@braap.org>) id 1b2pLc-00034N-7E
	for qemu-devel@nongnu.org; Tue, 17 May 2016 20:28:26 -0400
Received: from compute4.internal (compute4.nyi.internal [10.202.2.44])
	by mailout.nyi.internal (Postfix) with ESMTP id AFF0E20A8C
	for <qemu-devel@nongnu.org>; Tue, 17 May 2016 20:28:14 -0400 (EDT)
Date: Tue, 17 May 2016 20:28:14 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160518002814.GA25803@flamenco>
References: <1463196873-17737-1-git-send-email-cota@braap.org>
	<1463196873-17737-8-git-send-email-cota@braap.org>
	<573B5134.8060104@gmail.com>
	<66d14198-dab0-c72e-fe17-d022cff3feff@twiddle.net>
	<20160517200415.GD30174@flamenco> <573B7CFB.30002@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <573B7CFB.30002@gmail.com>
Subject: Re: [Qemu-devel] [PATCH v5 07/18] qemu-thread: add simple
 test-and-set spinlock
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: Sergey Fedorov <serge.fdrv@gmail.com>
Cc: Richard Henderson <rth@twiddle.net>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

On Tue, May 17, 2016 at 23:20:11 +0300, Sergey Fedorov wrote:
> On 17/05/16 23:04, Emilio G. Cota wrote:
(snip)
> > +/*
> > + * We might we tempted to use __atomic_test_and_set with __ATOMIC_ACQUIRE;
> > + * however, the documentation explicitly says that we should only pass
> > + * a boolean to it, so we use __sync_lock_test_and_set, which doesn't
> > + * have this limitation, and is documented to have acquire semantics.
> > + */
> > +#define atomic_test_and_set_acquire(ptr) __sync_lock_test_and_set(ptr, true)
> 
> So you are going to stick to *legacy* built-ins?

Why not? AFAIK the reason to avoid __sync primitives is that in most cases
they include barriers that callers might not necessarily need; __atomic's
allow for finer tuning, which is in general a good thing. However,
__sync_test_and_set has the exact semantics we need, without the limitations
documented for __atomic_test_and_set; so why not use it?

		E.