[PATCH] mm,compaction: serialize waitqueue_active() checks

[Davidlohr Bueso]

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.

Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
 mm/compaction.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index 949198d01260..fb0f87554eb9 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1950,6 +1950,13 @@ void wakeup_kcompactd(pg_data_t *pgdat, int order, int classzone_idx)
 	if (pgdat->kcompactd_max_order < order)
 		pgdat->kcompactd_max_order = order;
 
+	/*
+	 * Pairs with implicit barrier in wait_event_freezable()
+	 * such that wakeups are not missed in the lockless
+	 * waitqueue_active() call.
+	 */
+	smp_acquire__after_ctrl_dep();
+
 	if (pgdat->kcompactd_classzone_idx > classzone_idx)
 		pgdat->kcompactd_classzone_idx = classzone_idx;
 
-- 
2.6.6