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Message-ID: <20170914175313.GB811@linux-80c1.suse> Date: Thu, 14 Sep 2017 10:53:13 -0700 From: Davidlohr Bueso <dave@...olabs.net> To: akpm@...ux-foundation.org Cc: vbabka@...e.cz, parri.andrea@...il.com, linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de> Subject: Re: [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@...il.com> Signed-off-by: Davidlohr Bueso <dbueso@...e.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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