From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Srivatsa S. Bhat" Subject: Re: [PATCH 45/51] md, raid5: Fix CPU hotplug callback registration Date: Thu, 06 Feb 2014 15:35:22 +0530 Message-ID: <52F35E62.2060003@linux.vnet.ibm.com> References: <20140205220251.19080.92336.stgit@srivatsabhat.in.ibm.com> <20140205221244.19080.70910.stgit@srivatsabhat.in.ibm.com> <20140206121151.0cbdbaa1@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20140206121151.0cbdbaa1@notabene.brown> Sender: linux-kernel-owner@vger.kernel.org To: NeilBrown , oleg@redhat.com Cc: paulus@samba.org, rusty@rustcorp.com.au, peterz@infradead.org, tglx@linutronix.de, akpm@linux-foundation.org, mingo@kernel.org, paulmck@linux.vnet.ibm.com, tj@kernel.org, walken@google.com, ego@linux.vnet.ibm.com, linux@arm.linux.org.uk, linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org, stable@vger.kernel.org List-Id: linux-raid.ids On 02/06/2014 06:41 AM, NeilBrown wrote: > On Thu, 06 Feb 2014 03:42:45 +0530 "Srivatsa S. Bhat" > wrote: > >> From: Oleg Nesterov >> >> Subsystems that want to register CPU hotplug callbacks, as well as perform >> initialization for the CPUs that are already online, often do it as shown >> below: >> >> get_online_cpus(); >> >> for_each_online_cpu(cpu) >> init_cpu(cpu); >> >> register_cpu_notifier(&foobar_cpu_notifier); >> >> put_online_cpus(); >> >> This is wrong, since it is prone to ABBA deadlocks involving the >> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently >> with CPU hotplug operations). >> >> Interestingly, the raid5 code can actually prevent double initialization and >> hence can use the following simplified form of callback registration: >> >> register_cpu_notifier(&foobar_cpu_notifier); >> >> get_online_cpus(); >> >> for_each_online_cpu(cpu) >> init_cpu(cpu); >> >> put_online_cpus(); >> >> A hotplug operation that occurs between registering the notifier and calling >> get_online_cpus(), won't disrupt anything, because the code takes care to >> perform the memory allocations only once. >> >> So reorganize the code in raid5 this way to fix the deadlock with callback >> registration. >> >> Cc: Neil Brown >> Cc: linux-raid@vger.kernel.org >> Cc: stable@vger.kernel.org >> [Srivatsa: Fixed the unregister_cpu_notifier() deadlock, added the >> free_scratch_buffer() helper to condense code further and wrote the changelog.] >> Signed-off-by: Srivatsa S. Bhat >> --- [...] > > > Looks good, thanks. > Shall I wait for a signed-of-by from Oleg, then queue it through my md tree? > Sure, that sounds great, since this patch doesn't have any dependency. Thanks a lot! Oleg, it would be great if you could kindly add your S-O-B to this patch. Thanks! Regards, Srivatsa S. Bhat