The patch below does not apply to the 3.14-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <stable@xxxxxxxxxxxxxxx>. thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From c38d185d4af12e8be63ca4b6745d99449c450f12 Mon Sep 17 00:00:00 2001 From: Bart Van Assche <bvanassche@xxxxxxx> Date: Tue, 9 Dec 2014 16:58:35 +0100 Subject: [PATCH] blk-mq: Fix a race between bt_clear_tag() and bt_get() 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@xxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: Robert Elliott <elliott@xxxxxx> Cc: Ming Lei <ming.lei@xxxxxxxxxxxxx> Cc: Alexander Gordeev <agordeev@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> # v3.13+ Signed-off-by: Jens Axboe <axboe@xxxxxx> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c index 0f5e22a7971f..e47c4c75fd33 100644 --- 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; } @@ -357,11 +357,10 @@ static void bt_clear_tag(struct blk_mq_bitmap_tags *bt, unsigned int tag) 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) -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html