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

[Davidlohr Bueso]

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.and...@gmail.com>
Signed-off-by: Davidlohr Bueso <dbu...@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