mm: drop hotplug lock from lru_add_drain_all()

mm: drop hotplug lock from lru_add_drain_all()

Pulling cpu hotplug locks inside the mm core function like
lru_add_drain_all just asks for problems and the recent lockdep splat
[1] just proves this.  While the usage in that particular case might be
wrong we should avoid the locking as lru_add_drain_all() is used in many
places.  It seems that this is not all that hard to achieve actually.

We have done the same thing for drain_all_pages which is analogous by
commit a459eeb7b852 ("mm, page_alloc: do not depend on cpu hotplug locks
inside the allocator").  All we have to care about is to handle

      - the work item might be executed on a different cpu in worker from
        unbound pool so it doesn't run on pinned on the cpu

      - we have to make sure that we do not race with page_alloc_cpu_dead
        calling lru_add_drain_cpu

the first part is already handled because the worker calls lru_add_drain
which disables preemption when calling lru_add_drain_cpu on the local
cpu it is draining.  The later is true because page_alloc_cpu_dead is
called on the controlling CPU after the hotplugged CPU vanished
completely.

[1] http://lkml.kernel.org/r/089e0825eec8955c1f055c83d476@google.com

[add a cpu hotplug locking interaction as per tglx]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Thomas Gleixner <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>