Messages in this thread | | | Date | Wed, 3 Oct 2012 09:43:41 +0200 (CEST) | From | Jiri Kosina <> | Subject | Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()") |
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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
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