blk-mq: bitmap tag: fix race on blk_mq_bitmap_tags::wake_cnt
This piece of code in bt_clear_tag() function is racy:
bs = bt_wake_ptr(bt);
if (bs && atomic_dec_and_test(&bs->wait_cnt)) {
atomic_set(&bs->wait_cnt, bt->wake_cnt);
wake_up(&bs->wait);
}
Since nothing prevents bt_wake_ptr() from returning the very
same 'bs' address on multiple CPUs, the following scenario is
possible:
CPU1 CPU2
---- ----
0. bs = bt_wake_ptr(bt); bs = bt_wake_ptr(bt);
1. atomic_dec_and_test(&bs->wait_cnt)
2. atomic_dec_and_test(&bs->wait_cnt)
3. atomic_set(&bs->wait_cnt, bt->wake_cnt);
If the decrement in [1] yields zero then for some amount of time
the decrement in [2] results in a negative/overflow value, which
is not expected. The follow-up assignment in [3] overwrites the
invalid value with the batch value (and likely prevents the issue
from being severe) which is still incorrect and should be a lesser.
Cc: Ming Lei <tom.leiming@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
1 file changed