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 <pet...@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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