mm,compaction: serialize waitqueue_active() checks (for real)
authorDavidlohr Bueso <dave@stgolabs.net>
Tue, 3 Oct 2017 23:15:03 +0000 (16:15 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Wed, 4 Oct 2017 00:54:24 +0000 (17:54 -0700)
commit6818600ff094ca255a7fe31838ad50c29656c3c5
tree43ed936d9217cf7b097f8b44e55200844151e35d
parenta1b2289cef92ef0e9a92afcd2e1ea71d5bcaaf64
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>
mm/compaction.c