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Date: Fri, 1 Sep 2017 16:14:50 +1000 From: Nicholas Piggin <npiggin@...il.com> To: Ingo Molnar <mingo@...hat.com> Cc: Nicholas Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>, Paul McKenney <paulmck@...ux.vnet.ibm.com>, Linus Torvalds <torvalds@...ux-foundation.org>, linux-kernel@...r.kernel.org Subject: [PATCH] swait: add missing barrier to swake_up swake_up and swake_up_all test the swaitqueue outside the lock, but they are missing the barrier that would ensure visibility of a previous store that sets the wakeup condition with the load that tests the swaitqueue. This could lead to a lost wakeup if there is memory reordering. Fix this as prescribed by the waitqueue_active comments. Signed-off-by: Nicholas Piggin <npiggin@...il.com> -- I noticed this when chasing down that rcu hang bug (which turned out to not be anything of the sort). I might be missing something here and it's safe somehow, but if so then it should have a comment where it diverges from normal waitqueues. It looks like there's a few callers which are also testing swait_active before swake_up without a barrier which look wrong, so I must be missing something but I'm not sure what. Thanks, Nick --- kernel/sched/swait.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c index 3d5610dcce11..9056278001d9 100644 --- a/kernel/sched/swait.c +++ b/kernel/sched/swait.c @@ -33,6 +33,11 @@ void swake_up(struct swait_queue_head *q) { unsigned long flags; + /* + * See waitqueue_active() comments for checking waiters outside + * the lock. Same principle applies here. + */ + smp_mb(); if (!swait_active(q)) return; @@ -51,6 +56,11 @@ void swake_up_all(struct swait_queue_head *q) struct swait_queue *curr; LIST_HEAD(tmp); + /* + * See waitqueue_active() comments for checking waiters outside + * the lock. Same principle applies here. + */ + smp_mb(); if (!swait_active(q)) return; -- 2.13.3
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