md/raid5: Fix CPU hotplug callback registration
authorOleg Nesterov <oleg@redhat.com>
Wed, 5 Feb 2014 22:12:45 +0000 (03:42 +0530)
committerNeilBrown <neilb@suse.de>
Thu, 13 Feb 2014 02:46:45 +0000 (13:46 +1100)
commit789b5e0315284463617e106baad360cb9e8db3ac
tree3014756d06cac24b2e0692f0ab05a351b7323677
parent1877db75589a895bbdc4c4c3f23558e57b521141
md/raid5: Fix CPU hotplug callback registration

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: linux-raid@vger.kernel.org
Cc: stable@vger.kernel.org (v2.6.32+)
Fixes: 36d1c6476be51101778882897b315bd928c8c7b5
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
[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@linux.vnet.ibm.com>
Signed-off-by: NeilBrown <neilb@suse.de>
drivers/md/raid5.c