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@linux-80c1.suse
Fixes: 46acef048a6 (mm,compaction: serialize waitqueue_active() checks)
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Andrea Parri <parri.andrea@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/compaction.c b/mm/compaction.c
index fb548e4c7bd4b44f426a8e45c5230508c57d36e6..03d31a87534160f23eadb0aefeb48feca89191ab 100644 (file)
--- 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))