From 6818600ff094ca255a7fe31838ad50c29656c3c5 Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Tue, 3 Oct 2017 16:15:03 -0700 Subject: [PATCH] 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 Reported-by: Andrea Parri Cc: Vlastimil Babka Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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)) -- 2.20.1