Commit 6818600f authored Oct 03, 2017 by Davidlohr Bueso Committed by Linus Torvalds Oct 03, 2017

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: 46acef04 (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>

parent a1b2289c

mm/compaction.c

+5 −8

Original line number	Diff line number	Diff line
		@@ -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))

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