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). *Then* hotplug notifier fires up, succeeds obtaining hotplug lock, kmem_cache_destroy() calls rcu_barrier in the meantime, and blocks itself on the hotplug lock there. Please note that the get_online_cpus() call in kmem_cache_destroy() doesn't play *any* role in this scenario. -- Jiri Kosina SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
