On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote: > On 10/03/2012 01:13 PM, Jiri Kosina wrote: > > On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote: > > > >>>>> CPU 0 CPU 1 > >>>>> kmem_cache_destroy() > >>>> > >>>> What about the get_online_cpus() right here at CPU0 before > >>>> calling mutex_lock(slab_mutex)? How can the cpu_up() proceed > >>>> on CPU1?? I still don't get it... :( > >>>> > >>>> (kmem_cache_destroy() uses get/put_online_cpus() around acquiring > >>>> and releasing slab_mutex). > >>> > >>> The problem is that there is a CPU-hotplug notifier for slab, which > >>> establishes hotplug->slab. > >> > >> Agreed. > >> > >>> Then having kmem_cache_destroy() call > >>> rcu_barrier() under the lock > >> > >> Ah, that's where I disagree. kmem_cache_destroy() *cannot* proceed at > >> this point in time, because it has invoked get_online_cpus()! It simply > >> cannot be running past that point in the presence of a running hotplug > >> notifier! So, kmem_cache_destroy() should have been sleeping on the > >> hotplug lock, waiting for the notifier to release it, no? > > > > Please look carefully at the scenario again. kmem_cache_destroy() calls > > get_online_cpus() before the hotplug notifier even starts. Hence it has no > > reason to block there (noone is holding hotplug lock). > > > > Agreed. > > > *Then* hotplug notifier fires up, succeeds obtaining hotplug lock, > > Ah, that's the problem! The hotplug reader-writer synchronization is not just > via a simple mutex. Its a refcount underneath. If kmem_cache_destroy() > incremented > the refcount, the hotplug-writer (cpu_up) will release the hotplug lock > immediately > and try again. IOW, a hotplug-reader (kmem_cache_destroy()) and a > hotplug-writer > (cpu_up) can *NEVER* run concurrently. If they do, we are totally screwed! > > > Take a look at the hotplug lock acquire function at the writer side: > > static void cpu_hotplug_begin(void) > { > cpu_hotplug.active_writer = current; > > for (;;) { > mutex_lock(&cpu_hotplug.lock); > if (likely(!cpu_hotplug.refcount)) <================ This > one! > break; > __set_current_state(TASK_UNINTERRUPTIBLE); > mutex_unlock(&cpu_hotplug.lock); > schedule(); > } > }
I acutally just came to the same conclusion (7 hours of sleep later, the mind indeed seems to be brighter ... what a poet I am). Lockdep doesn't know about this semantics of cpu_hotplug_begin(), and therefore gets confused by the fact that mutual exclusion is actually achieved through the refcount instead of mutex (and the same apparently happened to me). So right, now I agree that the deadlock scenario I have come up with is indeed bogus (*), and we just have to annotate this fact to lockdep somehow. And I actually believe that moving the slab_mutex around in kmem_cache_destroy() is a good anotation (maybe worth a separate comment in the code), as it not only makes the lockdep false positive go away, but it also reduces the mutex hold time. (*) I have seen machine locking hard reproducibly, but that was only with additional Paul's patch, so I guess the lock order there actually was wrong Thanks! -- Jiri Kosina SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/