Re: [Qemu-devel] [PATCH v2 2/9] migration: Fix a potential issue

[Amit Shah]

On (Mon) 13 Jun 2016 [05:07:39], Li, Liang Z wrote:
> > > > > +static void wait_for_decompress_done(void) {
> > > > > +    int idx, thread_count;
> > > > > +
> > > > > +    if (!migrate_use_compression()) {
> > > > > +        return;
> > > > > +    }
> > > > > +
> > > > > +    thread_count = migrate_decompress_threads();
> > > > > +    qemu_mutex_lock(&decomp_done_lock);
> > > > > +    for (idx = 0; idx < thread_count; idx++) {
> > > > > +        while (!decomp_param[idx].done) {
> > > > > +            qemu_cond_wait(&decomp_done_cond,
> > &decomp_done_lock);
> > > > > +        }
> > > > > +    }
> > > > > +    qemu_mutex_unlock(&decomp_done_lock);
> > > >
> > > > Not sure how this works: in the previous patch, done is set to false
> > > > under the decomp_done_lock.  Here, we take the lock, and wait for done
> > to turn false.
> > > > That can't happen because this thread holds the lock.
> > > > My reading is this is going to lead to a deadlock.  What am I missing?
> > > >
> > >
> > > This is the typical usage of the QemuCond, actually, in
> > > qemu_cond_wait() , decomp_done_lock will be unlocked at first and then
> > > locked again before
> > > qemu_cond_wait() return.  So deadlock won't happen.
> > 
> > In qemu-thread-posix.c, I don't see such unlock/lock.
> > 
> > 
> >             Amit
> 
> I mean in the 'pthread_cond_wait()' which called by qemu_cond_wait().

Yes, OK - makes sense now.  Thanks, I'll continue the review.

                Amit