From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:34551)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1WIy9I-0004hf-1A
	for qemu-devel@nongnu.org; Thu, 27 Feb 2014 05:25:12 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1WIy99-0008OG-Jz
	for qemu-devel@nongnu.org; Thu, 27 Feb 2014 05:25:03 -0500
Received: from mail-wi0-x22a.google.com ([2a00:1450:400c:c05::22a]:47834)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <stefanha@gmail.com>) id 1WIy99-0008O6-CH
	for qemu-devel@nongnu.org; Thu, 27 Feb 2014 05:24:55 -0500
Received: by mail-wi0-f170.google.com with SMTP id hi5so443159wib.1
	for <qemu-devel@nongnu.org>; Thu, 27 Feb 2014 02:24:54 -0800 (PST)
Date: Thu, 27 Feb 2014 11:24:51 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20140227102451.GD30387@stefanha-thinkpad.redhat.com>
References: <1393348774-14663-1-git-send-email-stefanha@redhat.com>
	<1393348774-14663-2-git-send-email-stefanha@redhat.com>
	<530CD3FD.4030609@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <530CD3FD.4030609@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v2 1/2] iothread: stash thread ID away
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: Eric Blake <eblake@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, "Shergill,
	Gurinder" <gurinder.shergill@hp.com>, qemu-devel@nongnu.org, Luiz Capitulino <lcapitulino@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, "Vinod, Chegu" <chegu_vinod@hp.com>

On Tue, Feb 25, 2014 at 10:33:49AM -0700, Eric Blake wrote:
> On 02/25/2014 10:19 AM, Stefan Hajnoczi wrote:
> > Keep the thread ID around so we can report it via QMP.
> > 
> > There's only one problem: qemu_get_thread_id() (gettid() wrapper on
> > Linux) must be called from the thread itself.  There is no way to get
> > the thread ID outside the thread.
> > 
> > This patch uses a condvar to wait for iothread_run() to populate the
> > thread_id inside the thread.
> > 
> 
> > +
> > +    /* Wait for initialization to complete */
> > +    qemu_mutex_lock(&iothread->init_done_lock);
> > +    qemu_cond_wait(&iothread->init_done_cond,
> > +                   &iothread->init_done_lock);
> > +    qemu_mutex_unlock(&iothread->init_done_lock);
> 
> Generally, cond_wait needs to be done in a loop, where you also check
> the condition (in this case, that thread_id was actually set) - a
> pthread implementation is allowed to do spurious wakeups even if no
> other thread has managed to change the condition that you were waiting for.

You are right, POSIX is explicit about this:

"When using condition variables there is always a Boolean predicate
involving shared variables associated with each condition wait that is
true if the thread should proceed. Spurious wakeups from the
pthread_cond_timedwait() or pthread_cond_wait() functions may occur.
Since the return from pthread_cond_timedwait() or pthread_cond_wait()
does not imply anything about the value of this predicate, the predicate
should be re-evaluated upon such return."

I figured no other thread issues pthread_cond_signal/broadcast so
spurious wakeups cannot occur but I was wrong.

Stefan