All of
 help / color / mirror / Atom feed
* [PATCH] mm,compaction: serialize waitqueue_active() checks
@ 2017-01-09 15:25 ` Davidlohr Bueso
  0 siblings, 0 replies; 2+ messages in thread
From: Davidlohr Bueso @ 2017-01-09 15:25 UTC (permalink / raw)
  To: akpm; +Cc: vbabka, dave, linux-mm, linux-kernel, Davidlohr Bueso

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

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 <>
 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;

^ permalink raw reply	[flat|nested] 2+ messages in thread

end of thread, other threads:[~2017-01-09 15:25 UTC | newest]

Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2017-01-09 15:25 [PATCH] mm,compaction: serialize waitqueue_active() checks Davidlohr Bueso
2017-01-09 15:25 ` Davidlohr Bueso

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.