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Date: Tue, 9 Dec 2014 08:59:30 +0100 From: David Hildenbrand <dahi@...ux.vnet.ibm.com> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: linux-kernel@...r.kernel.org, heiko.carstens@...ibm.com, borntraeger@...ibm.com, rafael.j.wysocki@...el.com, peterz@...radead.org, oleg@...hat.com, bp@...e.de, jkosina@...e.cz Subject: Re: [PATCH v2] CPU hotplug: active_writer not woken up in some cases - deadlock > The compiler is within its rights to optimize the active_writer local > variable out of existence, thus re-introducing the possible race with > the writer that can pass a NULL pointer to wake_up_process(). So you > really need the ACCESS_ONCE() on the read from cpu_hotplug.active_writer. > Please see http://lwn.net/Articles/508991/ for more information why > this is absolutely required. You're absolutely right, saw your reply on the other patch just after I sent this version ... So if you agree with the change below, I'll send an updated version! > > > + if (unlikely(active_writer)) > > + wake_up_process(active_writer); > > cpuhp_lock_release(); > > return; > > } > > @@ -161,15 +167,17 @@ void cpu_hotplug_begin(void) > > cpuhp_lock_acquire(); > > for (;;) { > > mutex_lock(&cpu_hotplug.lock); > > + __set_current_state(TASK_UNINTERRUPTIBLE); > > You lost me on this one. How does this help? > > Thanx, Paul Imagine e.g. the following (simplified) scenario: CPU1 CPU2 ---------------------------------------------------------------------------- !mutex_trylock(&cpu_hotplug.lock) | | cpu_hotplug.puts_pending == 0 cpu_hotplug.puts_pending++; | | cpu_hotplug.refcount != 0 wake_up_process(active_writer) | __set_current_state(TASK_UNINTERRUPTIBLE); | schedule(); | /* will never be woken up */ Therefore we have to move the condition check inside the __set_current_state(TASK_UNINTERRUPTIBLE) -> schedule(); section to not miss any wake ups when the condition is satisfied. So wake_up_process() will either see TASK_RUNNING and do nothing or see TASK_UNINTERRUPTIBLE and set it to TASK_RUNNING, so schedule() will in fact be woken up again. Thanks a lot! David -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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