Re: [Qemu-devel] [PATCH 4/4] iothread: push gcontext earlier in the thread_fn

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 27, 2019 at 01:38:38PM +0000, Stefan Hajnoczi wrote:
> On Fri, Feb 22, 2019 at 02:57:24PM +0800, Peter Xu wrote:
> > On Fri, Feb 22, 2019 at 07:37:02AM +0100, Marc-André Lureau wrote:
> > > Hi
> > > 
> > > On Fri, Feb 22, 2019 at 4:14 AM Peter Xu <peterx@redhat.com> wrote:
> > > >
> > > > We were pushing the context until right before running the gmainloop.
> > > > Now since we have everything unconditionally, we can move this
> > > > earlier.
> > > >
> > > > One benefit is that now it's done even before init_done_sem, so as
> > > > long as the iothread user calls iothread_create() and completes, we
> > > > know that the thread stack is ready.
> > > >
> > > 
> > > This will change the default context in the iothread, for code running
> > > there. This may not be a good idea. Until now, only sources dispatched
> > > from iothread_get_g_main_context() would have default context
> > > associated to it.
> > > 
> > > I don't know if the current behaviour is intentional, but it has some
> > > logic. With this change, you may create hidden races, by changing the
> > > default context of sources to the iothread.
> > 
> > Yes I agree that the behavior will be changed in this patch that even
> > if the iothread user does not use the gcontext they'll also have the
> > context set.  I would think it should be ok because IMHO events hooked
> > onto the aio context should not depend on the gcontext, but indeed I'd
> > like to get some confirmation from others, especially the block layer.
> 
> I don't understand why Patch 4 is desirable.  The comment about
> init_done_sem isn't clear to me but I also wondered the same thing as
> Marc-André.
> 
> Can you explain why we should apply this patch?

Hi, Stefan,

The patch 4 itself does not help much for current QEMU, but it should
be required to replace the patch that Marc-Andre proposed below:

https://lists.gnu.org/archive/html/qemu-devel/2019-02/msg05460.html [1]

And IMHO patch 4 along with this whole series should be a cleaner
approach comparing to the one proposed in [1].  Here if my
understanding is correct the problem is that
g_main_context_push_thread_default() is really designed to be called
at the very beginning of a thread creation but not dynamically called
during the execution of a thread (prove is that it even does not have
any error to return when failed to acquire the context so the caller
will never know if it failed! see [2] below), in that sense this patch
4 can be seen as a tiny cleanup too.

g_main_context_push_thread_default (GMainContext *context)
{
  GQueue *stack;
  gboolean acquired_context;

  acquired_context = g_main_context_acquire (context);
  g_return_if_fail (acquired_context);  <------------- [2]

  ...
}

Regards,

-- 
Peter Xu