Hi On Mon, Mar 4, 2019 at 3:27 AM Peter Xu <pet...@redhat.com> wrote: > > On Fri, Mar 01, 2019 at 04:25:17PM +0000, Stefan Hajnoczi wrote: > > 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. > > Sure, I can add a comment above g_main_context_push_thread_default() > to emphasize why it's preferred at the entry. > > > > > 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. > > IIUC Paolo has answered this question (Message-ID: > <0faeceb2-68fa-59b0-48c3-b8e907b2a...@redhat.com>) - if the block > layer (or say, the explicit aio_poll in the iothread_run) does not use > the GMainContext at all then it should affect nothing, and with that > there should have no real functional change. >
It's a bold claim though, iothread isn't used only by the block layer. It's used also by colo in qemu tree, and I used to have a branch for virgl rendering for example. There might be other experimental work or usage I missed. Furthermore, even the block layer, and its various dependencies, is hard to review thoroughly. But it looks like it doesn't rely on glib main context, I agree. But I would get prepared for some weird (difficult to debug) regressions eventually.
