Re: [Qemu-devel] [PATCH v2 1/2] iothread: stash thread ID away

[Stefan Hajnoczi <stefanha@gmail.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