| From | Greg Kroah-Hartman <> | Subject | [PATCH 3.18 049/150] blk-mq: Fix a race between bt_clear_tag() and bt_get() | Date | Tue, 13 Jan 2015 23:22:00 -0800 |
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3.18-stable review patch. If anyone has any objections, please let me know.
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From: Bart Van Assche <bvanassche@acm.org>
commit c38d185d4af12e8be63ca4b6745d99449c450f12 upstream.
What we need is the following two guarantees: * Any thread that observes the effect of the test_and_set_bit() by __bt_get_word() also observes the preceding addition of 'current' to the appropriate wait list. This is guaranteed by the semantics of the spin_unlock() operation performed by prepare_and_wait(). Hence the conversion of test_and_set_bit_lock() into test_and_set_bit(). * The wait lists are examined by bt_clear() after the tag bit has been cleared. clear_bit_unlock() guarantees that any thread that observes that the bit has been cleared also observes the store operations preceding clear_bit_unlock(). However, clear_bit_unlock() does not prevent that the wait lists are examined before that the tag bit is cleared. Hence the addition of a memory barrier between clear_bit() and the wait list examination.
Signed-off-by: Bart Van Assche <bvanassche@acm.org> Cc: Christoph Hellwig <hch@lst.de> Cc: Robert Elliott <elliott@hp.com> Cc: Ming Lei <ming.lei@canonical.com> Cc: Alexander Gordeev <agordeev@redhat.com> Signed-off-by: Jens Axboe <axboe@fb.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
--- block/blk-mq-tag.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-)
--- a/block/blk-mq-tag.c +++ b/block/blk-mq-tag.c @@ -158,7 +158,7 @@ restart: return -1; } last_tag = tag + 1; - } while (test_and_set_bit_lock(tag, &bm->word)); + } while (test_and_set_bit(tag, &bm->word)); return tag; } @@ -342,11 +342,10 @@ static void bt_clear_tag(struct blk_mq_b struct bt_wait_state *bs; int wait_cnt; - /* - * The unlock memory barrier need to order access to req in free - * path and clearing tag bit - */ - clear_bit_unlock(TAG_TO_BIT(bt, tag), &bt->map[index].word); + clear_bit(TAG_TO_BIT(bt, tag), &bt->map[index].word); + + /* Ensure that the wait list checks occur after clear_bit(). */ + smp_mb(); bs = bt_wake_ptr(bt); if (!bs)
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