On Mon, Dec 18, 2023 at 10:32:07AM +0000, Daniel P. Berrangé wrote:
> On Fri, Dec 15, 2023 at 02:32:19AM +0800, Fima Shevrin wrote:
> > RPC client implementation uses the following paradigm. The critical
> > section is organized via virObjectLock(client)/virObjectUnlock(client)
> > braces. Though this is potentially problematic as
> > main thread: side thread:
> > virObjectUnlock(client);
> > virObjectLock(client);
> > g_main_loop_quit(client->eventLoop);
> > virObjectUnlock(client);
> > g_main_loop_run(client->eventLoop);
> >
> > This means in particular that is the main thread is executing very long
> > request like VM migration, the wakeup from the side thread could be
> > stuck until the main request will be fully completed.
>
> Ah, I understand this now. The flag set from g_main_loop_quit() is
> ignored and overwritten by g_main_loop_run(). So the interruption
> from the side thread never happens.
>
> Your approach to solving this is by delaying the virObjectUnlock
> call until g_main_loop_run is running, which is clever but I don't
> much like playing games with the mutex locking.
>
> We need a way to interrupt the main loop, even if it hasn't started
> running yet. The previous impl in libvirt used a pipe for this trick,
> effectively as a dummy event source that would be watched.
>
> I think we can do the same here, though without needing an actual
> pipe which causes Windows portability problems.
>
> Glib already has an internal mechansim for breaking out of poll(),
> via its (private) GWakeup object which encapsulates a pipe. We just
> need a way of triggering this mechanism.
>
> I believe this can be achieved using just an idle source ie
>
> static gboolean
> dummy_cb(void *opaque)
> {
Opps, would still need a g_main_loop_quit() call here
> return G_SOURCE_REMOVE;
> }
>
> ...
>
> GSource *dummy = g_idle_source_new()
> g_source_set_callback(dummy, dummy_cb, NULL, NULL);
> g_source_attach(dummy, client->eventCtx);
>
>
> The g_source_attach() method is what breaks the main loop out of
> its poll() sleep. If it wasn't currently in poll, it is effectively
> a no-op.
With regards,
Daniel
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