mm,compaction: serialize waitqueue_active() checks

Without a memory barrier, the following race can occur with a high-order
allocation:

wakeup_kcompactd(order == 1)  		     kcompactd()
  [L] waitqueue_active(kcompactd_wait)
						[S] prepare_to_wait_event(kcompactd_wait)
						[L] (kcompactd_max_order == 0)
  [S] kcompactd_max_order = order;		      schedule()

Where the waitqueue_active() check is speculatively re-ordered to before
setting the actual condition (max_order), not seeing the threads that's
going to block; making us miss a wakeup.  There are a couple of options
to fix this, including calling wq_has_sleepers() which adds a full
barrier, or unconditionally doing the wake_up_interruptible() and
serialize on the q->lock.  However, to make use of the control
dependency, we just need to add L->L guarantees.

While this bug is theoretical, there have been other offenders of the
lockless waitqueue_active() in the past -- this is also documented in
the call itself.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Davidlohr Bueso <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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