Re: [PATCH 3/3] rpc: Rework rpc notifications in main and side thread

On Fri, Dec 15, 2023 at 03:51:19PM +0100, Denis V. Lunev wrote: > On 12/15/23 14:48, Efim Shevrin wrote: >> *From:* Daniel P. Berrangé <berrange(a)redhat.com&gt; >> *Sent:* Friday, December 15, 2023 19:09 >> *To:* Efim Shevrin <efim.shevrin(a)virtuozzo.com&gt; >> *Cc:* devel(a)lists.libvirt.org <devel(a)lists.libvirt.org>; den(a)openvz.org >> <den(a)openvz.org&gt; >> *Subject:* Re: [PATCH 3/3] rpc: Rework rpc notifications in main and >> side thread >> [You don't often get email from berrange(a)redhat.com. Learn why this is >> important at https://aka.ms/LearnAboutSenderIdentification ] >> >> 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. >> Can you explain this in more detail, with call traces illustration >> for the two threads. You're not saying where the main thread is >> doing work with the 'client' lock hold for a long time. Generally >> the goal should be for the main thread to only hold the lock for >> a short time.  Also if the side thread is already holding a reference >> on 'client', then potentially we should consider if it is possible >> to terminate the event loop without acquiring the mutex, as GMainLoop >> protects itself wrt concurrent usage already, provided all threads >> hold a reference directly or indirectly. > At our opinion the problem here is missed wakeup from > the side thread to the main thread. Hmmm, what platform are you seeing problems on ? Are you still targetting a very old RHEL-7 platform ? I vaguely recall there are/weere some old glib bugs in the main loop with threads that could be applicable. With regards, Daniel