From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754779AbaBSVmX (ORCPT ); Wed, 19 Feb 2014 16:42:23 -0500 Received: from g9t1613g.houston.hp.com ([15.240.0.71]:49488 "EHLO g9t1613g.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753449AbaBSVmW (ORCPT ); Wed, 19 Feb 2014 16:42:22 -0500 Message-ID: <1392845686.6784.3.camel@misato.fc.hp.com> Subject: Re: [PATCH v2 02/52] CPU hotplug: Provide lockless versions of callback registration functions From: Toshi Kani To: "Srivatsa S. Bhat" Cc: paulus@samba.org, oleg@redhat.com, mingo@kernel.org, rusty@rustcorp.com.au, peterz@infradead.org, tglx@linutronix.de, akpm@linux-foundation.org, paulmck@linux.vnet.ibm.com, tj@kernel.org, walken@google.com, ego@linux.vnet.ibm.com, linux@arm.linux.org.uk, rjw@rjwysocki.net, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Date: Wed, 19 Feb 2014 14:34:46 -0700 In-Reply-To: <20140214074922.22701.17949.stgit@srivatsabhat.in.ibm.com> References: <20140214074750.22701.47330.stgit@srivatsabhat.in.ibm.com> <20140214074922.22701.17949.stgit@srivatsabhat.in.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.8.5 (3.8.5-2.fc19) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2014-02-14 at 13:19 +0530, Srivatsa S. Bhat wrote: > The following method of CPU hotplug callback registration is not safe > due to the possibility of an ABBA deadlock involving the cpu_add_remove_lock > and the cpu_hotplug.lock. > > get_online_cpus(); > > for_each_online_cpu(cpu) > init_cpu(cpu); > > register_cpu_notifier(&foobar_cpu_notifier); > > put_online_cpus(); > > The deadlock is shown below: > > CPU 0 CPU 1 > ----- ----- > > Acquire cpu_hotplug.lock > [via get_online_cpus()] > > CPU online/offline operation > takes cpu_add_remove_lock > [via cpu_maps_update_begin()] > > > Try to acquire > cpu_add_remove_lock > [via register_cpu_notifier()] > > > CPU online/offline operation > tries to acquire cpu_hotplug.lock > [via cpu_hotplug_begin()] > > > *** DEADLOCK! *** > > The problem here is that callback registration takes the locks in one order > whereas the CPU hotplug operations take the same locks in the opposite order. > To avoid this issue and to provide a race-free method to register CPU hotplug > callbacks (along with initialization of already online CPUs), introduce new > variants of the callback registration APIs that simply register the callbacks > without holding the cpu_add_remove_lock during the registration. That way, > we can avoid the ABBA scenario. However, we will need to hold the > cpu_add_remove_lock throughout the entire critical section, to protect updates > to the callback/notifier chain. > > This can be achieved by writing the callback registration code as follows: > > cpu_maps_update_begin(); [ or cpu_notifier_register_begin(); see below ] > > for_each_online_cpu(cpu) > init_cpu(cpu); > > /* This doesn't take the cpu_add_remove_lock */ > __register_cpu_notifier(&foobar_cpu_notifier); > > cpu_maps_update_done(); [ or cpu_notifier_register_done(); see below ] > > Note that we can't use get_online_cpus() here instead of cpu_maps_update_begin() > because the cpu_hotplug.lock is dropped during the invocation of CPU_POST_DEAD > notifiers, and hence get_online_cpus() cannot provide the necessary > synchronization to protect the callback/notifier chains against concurrent > reads and writes. On the other hand, since the cpu_add_remove_lock protects > the entire hotplug operation (including CPU_POST_DEAD), we can use > cpu_maps_update_begin/done() to guarantee proper synchronization. > > Also, since cpu_maps_update_begin/done() is like a super-set of > get/put_online_cpus(), the former naturally protects the critical sections > from concurrent hotplug operations. > > Since the names cpu_maps_update_begin/done() don't make much sense in CPU > hotplug callback registration scenarios, we'll introduce new APIs named > cpu_notifier_register_begin/done() and map them to cpu_maps_update_begin/done(). > > In summary, introduce the lockless variants of un/register_cpu_notifier() and > also export the cpu_notifier_register_begin/done() APIs for use by modules. > This way, we provide a race-free way to register hotplug callbacks as well as > perform initialization for the CPUs that are already online. > > Cc: Thomas Gleixner > Cc: Toshi Kani > Cc: "Rafael J. Wysocki" > Cc: Andrew Morton > Cc: Peter Zijlstra > Cc: Ingo Molnar > Acked-by: Oleg Nesterov > Signed-off-by: Srivatsa S. Bhat Acked-by: Toshi Kani Thanks, -Toshi