Messages in this thread | | | Date | Wed, 10 Dec 2014 18:50:55 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH v4] CPU hotplug: active_writer not woken up in some cases - deadlock |
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On 12/10, David Hildenbrand wrote: > > @@ -127,20 +119,16 @@ void put_online_cpus(void) > { > if (cpu_hotplug.active_writer == current) > return; > - if (!mutex_trylock(&cpu_hotplug.lock)) { > - atomic_inc(&cpu_hotplug.puts_pending); > - cpuhp_lock_release(); > - return; > - } > - > - if (WARN_ON(!cpu_hotplug.refcount)) > - cpu_hotplug.refcount++; /* try to fix things up */ > > - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) > - wake_up_process(cpu_hotplug.active_writer); > - mutex_unlock(&cpu_hotplug.lock); > - cpuhp_lock_release(); > + if (atomic_dec_and_test(&cpu_hotplug.refcount) && > + waitqueue_active(&cpu_hotplug.wq)) > + wake_up(&cpu_hotplug.wq);
OK, waitqueue_active() looks safe... prepare_to_wait() has a barrier.
> void cpu_hotplug_begin(void) > { > + DEFINE_WAIT(wait); > + > cpu_hotplug.active_writer = current; > > - cpuhp_lock_acquire(); > for (;;) { > + cpuhp_lock_acquire();
not sure I understand why did you move cpuhp_lock_acquire() into the loop, but this is minor.
> mutex_lock(&cpu_hotplug.lock); > - apply_puts_pending(1); > - if (likely(!cpu_hotplug.refcount)) > + prepare_to_wait(&cpu_hotplug.wq, &wait, TASK_UNINTERRUPTIBLE); > + if (likely(!atomic_read(&cpu_hotplug.refcount))) > break; > - __set_current_state(TASK_UNINTERRUPTIBLE); > mutex_unlock(&cpu_hotplug.lock); > + cpuhp_lock_release(); > schedule(); > } > + > + finish_wait(&cpu_hotplug.wq, &wait); > }
This is subjective, but how about
static bool xxx(void) { mutex_lock(&cpu_hotplug.lock); if (atomic_read(&cpu_hotplug.refcount) == 0) return true; mutex_unlock(&cpu_hotplug.lock); return false; }
void cpu_hotplug_begin(void) { cpu_hotplug.active_writer = current;
cpuhp_lock_acquire(); wait_event(&cpu_hotplug.wq, xxx()); }
instead?
Oleg.
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