From: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Subject: mm,compaction: serialize waitqueue_active() checks (for real)
Andrea brought to my attention that the L->{L,S} guarantees are completely
bogus for this case. I was looking at the diagram, from the offending
commit, when that _is_ the race, we had the load reordered already.
What we need is at least S->L semantics, thus simply use wq_has_sleeper()
to serialize the call for good.
Link: http://lkml.kernel.org/r/20170914175313.GB811@xxxxxxxxxxxxxxx
Fixes: 46acef048a6 (mm,compaction: serialize waitqueue_active() checks)
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Reported-by: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/compaction.c | 13 +++++--------
1 file changed, 5 insertions(+), 8 deletions(-)
diff -puN mm/compaction.c~mmcompaction-serialize-waitqueue_active-checks-for-real mm/compaction.c
--- a/mm/compaction.c~mmcompaction-serialize-waitqueue_active-checks-for-real
+++ a/mm/compaction.c
@@ -1999,17 +1999,14 @@ void wakeup_kcompactd(pg_data_t *pgdat,
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;
- if (!waitqueue_active(&pgdat->kcompactd_wait))
+ /*
+ * Pairs with implicit barrier in wait_event_freezable()
+ * such that wakeups are not missed.
+ */
+ if (!wq_has_sleeper(&pgdat->kcompactd_wait))
return;
if (!kcompactd_node_suitable(pgdat))
_
--
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