Re: [Qemu-devel] [RFC v3 01/27] char-io: fix possible race on IOWatchPoll

[Peter Xu <peterx@redhat.com>]

On Mon, Nov 13, 2017 at 04:52:11PM +0000, Stefan Hajnoczi wrote:
> On Mon, Nov 06, 2017 at 05:46:17PM +0800, Peter Xu wrote:
> > This is not a problem if we are only having one single loop thread like
> > before.  However, after per-monitor thread is introduced, this is not
> > true any more, and the race can happen.
> > 
> > The race can be triggered with "make check -j8" sometimes:
> 
> Please mention a specific test case that fails.

It was any of the check-qtest-$(TARGET)s that failed.  I'll mention
that in next post.

> 
> > 
> >   qemu-system-x86_64: /root/git/qemu/chardev/char-io.c:91:
> >   io_watch_poll_finalize: Assertion `iwp->src == NULL' failed.
> > 
> > This patch keeps the reference for the watch object when creating in
> > io_add_watch_poll(), so that the object will never be released in the
> > context main loop, especially when the context loop is running in
> > another standalone thread.  Meanwhile, when we want to remove the watch
> > object, we always first detach the watch object from its owner context,
> > then we continue with the cleanup.
> > 
> > Without this patch, calling io_remove_watch_poll() in main loop thread
> > is not thread-safe, since the other per-monitor thread may be modifying
> > the watch object at the same time.
> > 
> > Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  chardev/char-io.c | 16 ++++++++++++++--
> >  1 file changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/chardev/char-io.c b/chardev/char-io.c
> > index f81052481a..50b5bac704 100644
> > --- a/chardev/char-io.c
> > +++ b/chardev/char-io.c
> > @@ -122,7 +122,6 @@ GSource *io_add_watch_poll(Chardev *chr,
> >      g_free(name);
> >  
> >      g_source_attach(&iwp->parent, context);
> > -    g_source_unref(&iwp->parent);
> >      return (GSource *)iwp;
> >  }
> >  
> > @@ -131,12 +130,25 @@ static void io_remove_watch_poll(GSource *source)
> >      IOWatchPoll *iwp;
> >  
> >      iwp = io_watch_poll_from_source(source);
> > +
> > +    /*
> > +     * Here the order of destruction really matters.  We need to first
> > +     * detach the IOWatchPoll object from the context (which may still
> > +     * be running in another loop thread), only after that could we
> > +     * continue to operate on iwp->src, or there may be race condition
> > +     * between current thread and the context loop thread.
> > +     *
> > +     * Let's blame the glib bug mentioned in commit 2b316774f6
> > +     * ("qemu-char: do not operate on sources from finalize
> > +     * callbacks") for this extra complexity.
> 
> I don't understand how this bug is to blame.  Isn't the problem here a
> race condition between two QEMU threads?

Yes, it is.

The problem is, we won't have the race condition if glib does not have
that bug mentioned.  Then the thread running GMainContext will have
full control of iwp->src destruction, and destruction of it would be
fairly straightforward (unref iwp->src in IOWatchPoll destructor).
Now IIUC we are doing this in a hacky way, say, we destroy iwp->src
explicitly from main thread before quitting (see [1] below, the whole
if clause).

> 
> Why are two threads accessing the watch at the same time?

Here is how I understand:

Firstly we need to tackle with that bug, by an explicit destruction of
iwp->src below; meanwhile when we are destroying it, the GMainContext
can still be running somewhere (it's not happening in current series
since I stopped iothread earlier than this point, however it can still
happen if in the future we don't do that), then we possibly want this
patch.

Again, without this patch, current series should work; however I do
hope this patch can be in, in case someday we want to provide complete
thread safety for Chardevs (now it is not really thread-safe).

> 
> > +     */
> > +    g_source_destroy(&iwp->parent);
> >      if (iwp->src) {
> >          g_source_destroy(iwp->src);
> >          g_source_unref(iwp->src);
> >          iwp->src = NULL;
> >      }

[1]

> > -    g_source_destroy(&iwp->parent);
> > +    g_source_unref(&iwp->parent);
> >  }
> >  
> >  void remove_fd_in_watch(Chardev *chr)
> > -- 
> > 2.13.5
> > 

Thanks,

-- 
Peter Xu