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SubjectRe: [PATCH] mm,compaction: serialize waitqueue_active() checks (for real)
On Wed, 13 Sep 2017, Davidlohr Bueso wrote:

>- /*
>- * 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();

sorry, forgot to delete this. Here's v2.

-----8<-----------------------------------------------------
[PATCH v2] 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.

Fixes: 46acef048a6 (mm,compaction: serialize waitqueue_active() checks)
Reported-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
mm/compaction.c | 13 +++++--------
1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index fb548e4c7bd4..03d31a875341 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1999,17 +1999,14 @@ 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;

- 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))
--
2.12.0
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