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

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Feb 28, 2019 at 01:58:56PM +0800, Peter Xu wrote:
> 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]
> 
>   ...
> }

I see.  This explains why you want to call it early.  If you're worried
about that then there should also be a comment warning people that this
must happen first before anything implicitly uses the thread's
GMainContext.

What about Marc-André's concern about the change in behavior?  Now this
thread is associated with the GMainContext that isn't processed at in
aio_poll().  Previously the default main context would be used.

Stefan

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