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| H A D | sbitmap.c | diff c854ab5773be1c1a0d3cef0c3a3261f2c48ab7f8 Mon May 14 20:17:31 CEST 2018 Jens Axboe <axboe@kernel.dk> sbitmap: fix race in wait batch accounting
If we have multiple callers of sbq_wake_up(), we can end up in a
situation where the wait_cnt will continually go more and more
negative. Consider the case where our wake batch is 1, hence
wait_cnt will start out as 1.
wait_cnt == 1
CPU0 CPU1
atomic_dec_return(), cnt == 0
atomic_dec_return(), cnt == -1
cmpxchg(-1, 0) (succeeds)
[wait_cnt now 0]
cmpxchg(0, 1) (fails)
This ends up with wait_cnt being 0, we'll wakeup immediately
next time. Going through the same loop as above again, and
we'll have wait_cnt -1.
For the case where we have a larger wake batch, the only
difference is that the starting point will be higher. We'll
still end up with continually smaller batch wakeups, which
defeats the purpose of the rolling wakeups.
Always reset the wait_cnt to the batch value. Then it doesn't
matter who wins the race. But ensure that whomever does win
the race is the one that increments the ws index and wakes up
our batch count, loser gets to call __sbq_wake_up() again to
account his wakeups towards the next active wait state index.
Fixes: 6c0ca7ae292a ("sbitmap: fix wakeup hang after sbq resize")
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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