On Tue, 2 Oct 2012, Jiri Kosina wrote: > > > 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is the first bad commit > > > commit 1331e7a1bbe1f11b19c4327ba0853bee2a606543 > > > Author: Paul E. McKenney <paul.mcken...@linaro.org> > > > Date: Thu Aug 2 17:43:50 2012 -0700 > > > > > > rcu: Remove _rcu_barrier() dependency on __stop_machine() > > > > > > Currently, _rcu_barrier() relies on preempt_disable() to prevent > > > any CPU from going offline, which in turn depends on CPU hotplug's > > > use of __stop_machine(). > > > > > > This patch therefore makes _rcu_barrier() use get_online_cpus() to > > > block CPU-hotplug operations. This has the added benefit of removing > > > the need for _rcu_barrier() to adopt callbacks: Because CPU-hotplug > > > operations are excluded, there can be no callbacks to adopt. This > > > commit simplifies the code accordingly. > > > > > > Signed-off-by: Paul E. McKenney <paul.mcken...@linaro.org> > > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > > > Reviewed-by: Josh Triplett <j...@joshtriplett.org> > > > == > > > > > > is causing lockdep to complain (see the full trace below). I haven't yet > > > had time to analyze what exactly is happening, and probably will not have > > > time to do so until tomorrow, so just sending this as a heads-up in case > > > anyone sees the culprit immediately. > > > > Hmmm... Does the following patch help? It swaps the order in which > > rcu_barrier() acquires the hotplug and rcu_barrier locks. > > It changed the report slightly (see for example the change in possible > unsafe locking scenario, rcu_sched_state.barrier_mutex vanished and it's > now directly about cpu_hotplug.lock). With the patch applied I get > > > > ====================================================== > [ INFO: possible circular locking dependency detected ] > 3.6.0-03888-g3f99f3b #145 Not tainted
And it really seems valid. kmem_cache_destroy() calls rcu_barrier() with slab_mutex locked, which introduces slab_mutex -> cpu_hotplug.lock dependency (through rcu_barrier() -> _rcu_barrier() -> get_online_cpus()). On the other hand, _cpu_up() acquires cpu_hotplug.lock through cpu_hotplug_begin(), and with this lock held cpuup_callback() notifier gets called, which acquires slab_mutex. This gives the reverse dependency, i.e. deadlock scenario is valid one. 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is triggering this, because before that, there was no slab_mutex -> cpu_hotplug.lock dependency. Simply put, the commit causes get_online_cpus() to be called with slab_mutex held, which is invalid. -- 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/
