From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:38445)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1VFKEF-000256-Fl
	for qemu-devel@nongnu.org; Fri, 30 Aug 2013 04:38:59 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1VFKE7-0003mw-1w
	for qemu-devel@nongnu.org; Fri, 30 Aug 2013 04:38:51 -0400
Received: from mail-qe0-x233.google.com ([2607:f8b0:400d:c02::233]:42979)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <paolo.bonzini@gmail.com>) id 1VFKE6-0003ms-Sq
	for qemu-devel@nongnu.org; Fri, 30 Aug 2013 04:38:42 -0400
Received: by mail-qe0-f51.google.com with SMTP id cy11so805929qeb.38
	for <qemu-devel@nongnu.org>; Fri, 30 Aug 2013 01:38:42 -0700 (PDT)
Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <52205A0B.1040606@redhat.com>
Date: Fri, 30 Aug 2013 10:38:35 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1377849572-42883-1-git-send-email-cornelia.huck@de.ibm.com>
	<1377849572-42883-2-git-send-email-cornelia.huck@de.ibm.com>
In-Reply-To: <1377849572-42883-2-git-send-email-cornelia.huck@de.ibm.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH] s390-ccw.img: Fix waiting for virtio
	processing.
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: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>, Alexander Graf <agraf@suse.de>, Dominik Dingel <dingel@linux.vnet.ibm.com>, qemu-devel <qemu-devel@nongnu.org>

Il 30/08/2013 09:59, Cornelia Huck ha scritto:
> +    vr->used_idx = vr->used->idx;
> +    /*
> +     * Wait until the used index has moved.
> +     * Note: This is only race-free if the host doesn't start draining
> +     * the queue out of its own accord.
> +     */
> +    while (vr->used->idx == vr->used_idx) {

Can you instead initialize vr->used_idx in virtio_setup_block, and then
update it _after_ the while loop?  This would be race-free.

Paolo