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authorDavidlohr Bueso <dave@stgolabs.net>2017-02-22 15:44:55 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2017-02-22 16:41:29 -0800
commit46acef048a6568ba490f636db8682a1461ed223c (patch)
tree7efc7b456db2e0b3df833f895d409bf20efa8646 /mm/compaction.c
parentb92df1de5d289c0b5d653e72414bf0850b8511e0 (diff)
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/1483975528-24342-1-git-send-email-dave@stgolabs.net Signed-off-by: Davidlohr Bueso <dbueso@suse.de> Cc: Vlastimil Babka <vbabka@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/compaction.c')
-rw-r--r--mm/compaction.c7
1 files changed, 7 insertions, 0 deletions
diff --git a/mm/compaction.c b/mm/compaction.c
index c6178bbd3e04..0aa2757399ee 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1966,6 +1966,13 @@ 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;