On Fri, May 15, 2015 at 9:04 AM, Andrea Arcangeli [off-list ref] wrote:

To fix it I added this along a comment:

Ok, this looks good as a explanation/fix for the races (and also as an
example of my worry about waitqueue_active() use in general).

However, it now makes me suspect that the optimistic "let's check if
they are even active" may not be worth it any more. You're adding a
"smp_mb()" in order to avoid taking the real lock. Although I guess
there are two locks there (one for each wait-queue) so maybe it's
worth it.

                    Linus

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