Commit c38d185d authored Dec 09, 2014 by Bart Van Assche Committed by Jens Axboe Dec 09, 2014

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@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>
Cc: <stable@vger.kernel.org> # v3.13+
Signed-off-by: Jens Axboe <axboe@fb.com>

parent 9e98e9d7

block/blk-mq-tag.c

+5 −6

Original line number	Diff line number	Diff line
		@@ -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)