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mm,compaction: serialize waitqueue_active() checks (for real)

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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: 46acef048a (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>
This commit is contained in:
Davidlohr Bueso 2017-10-03 16:15:03 -07:00 committed by Linus Torvalds
parent a1b2289cef
commit 6818600ff0
1 changed files with 5 additions and 8 deletions
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13
mm/compaction.c
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@ -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))