[PATCH 3.16.y-ckt 103/216] blk-mq: Fix a race between bt_clear_tag() and bt_get()

Luis Henriques
2015-01-12T18:09:39Z

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: Luis Henriques <luis.henriques@canonical.com>
---
block/blk-mq-tag.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index a1bc33974fc4..1caaa7c4605d 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;
}
@@ -342,11 +342,10 @@ 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)
--
2.1.4