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Message-ID: <52F35E62.2060003@linux.vnet.ibm.com> Date: Thu, 06 Feb 2014 15:35:22 +0530 From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> To: NeilBrown <neilb@...e.de>, oleg@...hat.com CC: paulus@...ba.org, rusty@...tcorp.com.au, peterz@...radead.org, tglx@...utronix.de, akpm@...ux-foundation.org, mingo@...nel.org, paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com, ego@...ux.vnet.ibm.com, linux@....linux.org.uk, linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org, stable@...r.kernel.org Subject: Re: [PATCH 45/51] md, raid5: Fix CPU hotplug callback registration On 02/06/2014 06:41 AM, NeilBrown wrote: > On Thu, 06 Feb 2014 03:42:45 +0530 "Srivatsa S. Bhat" > <srivatsa.bhat@...ux.vnet.ibm.com> wrote: > >> From: Oleg Nesterov <oleg@...hat.com> >> >> 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 <neilb@...e.de> >> Cc: linux-raid@...r.kernel.org >> Cc: stable@...r.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 <srivatsa.bhat@...ux.vnet.ibm.com> >> --- [...] > > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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