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Re: [Qemu-devel] [PATCH 4/4] iothread: push gcontext earlier in the thre
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [PATCH 4/4] iothread: push gcontext earlier in the thread_fn |
Date: |
Thu, 28 Feb 2019 13:58:56 +0800 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
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 <address@hidden> 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
- [Qemu-devel] [PATCH 3/4] iothread: create main loop unconditionally, (continued)
Re: [Qemu-devel] [PATCH 0/4] iothread: create gcontext unconditionally, Paolo Bonzini, 2019/02/22