Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753318Ab2JCDmQ (ORCPT ); Tue, 2 Oct 2012 23:42:16 -0400 Received: from e38.co.us.ibm.com ([32.97.110.159]:52093 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753257Ab2JCDmP (ORCPT ); Tue, 2 Oct 2012 23:42:15 -0400 Date: Tue, 2 Oct 2012 20:41:38 -0700 From: "Paul E. McKenney" To: Jiri Kosina Cc: Christoph Lameter , Pekka Enberg , "Paul E. McKenney" , Josh Triplett , linux-kernel@vger.kernel.org, "Srivatsa S. Bhat" , linux-mm@kvack.org Subject: Re: [PATCH] mm, slab: release slab_mutex earlier in kmem_cache_destroy() (was Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()")) Message-ID: <20121003034138.GA13192@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20121002170149.GC2465@linux.vnet.ibm.com> <20121002233138.GD2465@linux.vnet.ibm.com> <20121003001530.GF2465@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12100303-5518-0000-0000-000008188250 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6040 Lines: 167 On Wed, Oct 03, 2012 at 02:45:30AM +0200, Jiri Kosina wrote: > On Tue, 2 Oct 2012, Paul E. McKenney wrote: > > > On Wed, Oct 03, 2012 at 01:48:21AM +0200, Jiri Kosina wrote: > > > On Tue, 2 Oct 2012, Paul E. McKenney wrote: > > > > > > > Indeed. Slab seems to be doing an rcu_barrier() in a CPU hotplug > > > > notifier, which doesn't sit so well with rcu_barrier() trying to exclude > > > > CPU hotplug events. I could go back to the old approach, but it is > > > > significantly more complex. I cannot say that I am all that happy about > > > > anyone calling rcu_barrier() from a CPU hotplug notifier because it > > > > doesn't help CPU hotplug latency, but that is a separate issue. > > > > > > > > But the thing is that rcu_barrier()'s assumptions work just fine if either > > > > (1) it excludes hotplug operations or (2) if it is called from a hotplug > > > > notifier. You see, either way, the CPU cannot go away while rcu_barrier() > > > > is executing. So the right way to resolve this seems to be to do the > > > > get_online_cpus() only if rcu_barrier() is -not- executing in the context > > > > of a hotplug notifier. Should be fixable without too much hassle... > > > > > > Sorry, I don't think I understand what you are proposing just yet. > > > > > > If I understand it correctly, you are proposing to introduce some magic > > > into _rcu_barrier() such as (pseudocode of course): > > > > > > if (!being_called_from_hotplug_notifier_callback) > > > get_online_cpus() > > > > > > How does that protect from the scenario I've outlined before though? > > > > > > CPU 0 CPU 1 > > > kmem_cache_destroy() > > > mutex_lock(slab_mutex) > > > _cpu_up() > > > cpu_hotplug_begin() > > > mutex_lock(cpu_hotplug.lock) > > > rcu_barrier() > > > _rcu_barrier() > > > get_online_cpus() > > > mutex_lock(cpu_hotplug.lock) > > > (blocks, CPU 1 has the mutex) > > > __cpu_notify() > > > mutex_lock(slab_mutex) > > > > > > CPU 0 grabs both locks anyway (it's not running from notifier callback). > > > CPU 1 grabs both locks as well, as there is no _rcu_barrier() being called > > > from notifier callback either. > > > > > > What did I miss? > > > > You didn't miss anything, I was suffering a failure to read carefully. > > > > So my next stupid question is "Why can't kmem_cache_destroy drop > > slab_mutex early?" like the following: > > > > void kmem_cache_destroy(struct kmem_cache *cachep) > > { > > BUG_ON(!cachep || in_interrupt()); > > > > /* Find the cache in the chain of caches. */ > > get_online_cpus(); > > mutex_lock(&slab_mutex); > > /* > > * the chain is never empty, cache_cache is never destroyed > > */ > > list_del(&cachep->list); > > if (__cache_shrink(cachep)) { > > slab_error(cachep, "Can't free all objects"); > > list_add(&cachep->list, &slab_caches); > > mutex_unlock(&slab_mutex); > > put_online_cpus(); > > return; > > } > > mutex_unlock(&slab_mutex); > > > > if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU)) > > rcu_barrier(); > > > > __kmem_cache_destroy(cachep); > > put_online_cpus(); > > } > > > > Or did I miss some reason why __kmem_cache_destroy() needs that lock? > > Looks to me like it is just freeing now-disconnected memory. > > Good question. I believe it should be safe to drop slab_mutex earlier, as > cachep has already been unlinked. I am adding slab people and linux-mm to > CC (the whole thread on LKML can be found at > https://lkml.org/lkml/2012/10/2/296 for reference). > > How about the patch below? Pekka, Christoph, please? > > It makes the lockdep happy again, and obviously removes the deadlock (I > tested it). You can certainly add my Reviewed-by, for whatever that is worth. ;-) BTW, with this patch, are you able to dispense with my earlier patch, or is it still needed? Thanx, Paul > From: Jiri Kosina > Subject: mm, slab: release slab_mutex earlier in kmem_cache_destroy() > > Commit 1331e7a1bbe1 ("rcu: Remove _rcu_barrier() dependency on > __stop_machine()") introduced slab_mutex -> cpu_hotplug.lock > dependency through kmem_cache_destroy() -> rcu_barrier() -> > _rcu_barrier() -> get_online_cpus(). > > This opens a possibilty for deadlock: > > CPU 0 CPU 1 > kmem_cache_destroy() > mutex_lock(slab_mutex) > _cpu_up() > cpu_hotplug_begin() > mutex_lock(cpu_hotplug.lock) > rcu_barrier() > _rcu_barrier() > get_online_cpus() > mutex_lock(cpu_hotplug.lock) > (blocks, CPU 1 has the mutex) > __cpu_notify() > mutex_lock(slab_mutex) > > It turns out that slab's kmem_cache_destroy() might release slab_mutex > earlier before calling out to rcu_barrier(), as cachep has already been > unlinked. > > This patch removes the AB-BA dependency by calling rcu_barrier() with > slab_mutex already unlocked. > > Signed-off-by: Jiri Kosina > --- > mm/slab.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/mm/slab.c b/mm/slab.c > index 1133911..693c7cb 100644 > --- a/mm/slab.c > +++ b/mm/slab.c > @@ -2801,12 +2801,12 @@ void kmem_cache_destroy(struct kmem_cache *cachep) > put_online_cpus(); > return; > } > + mutex_unlock(&slab_mutex); > > if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU)) > rcu_barrier(); > > __kmem_cache_destroy(cachep); > - mutex_unlock(&slab_mutex); > put_online_cpus(); > } > EXPORT_SYMBOL(kmem_cache_destroy); > > -- > Jiri Kosina > SUSE Labs > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/