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authorKosuke Tatsukawa <tatsu@ab.jp.nec.com>2015-10-09 00:35:38 +0000
committerJens Axboe <axboe@fb.com>2015-10-09 10:52:46 -0600
commit8ee1b7b9d8761a7b1a0f37a6e596b5315d27d2f2 (patch)
tree909a288ceefca515fe3a03eb183c1a2c5e3044d3
parentfd48ca384960c831d4fae4636282e422e140ca7c (diff)
blk-mq: fix waitqueue_active without memory barrier in block/blk-mq-tag.c
blk_mq_tag_update_depth() seems to be missing a memory barrier which might cause the waker to not notice the waiter and fail to send a wake_up as in the following figure. blk_mq_tag_update_depth bt_get ------------------------------------------------------------------------ if (waitqueue_active(&bs->wait)) /* The CPU might reorder the test for the waitqueue up here, before prior writes complete */ prepare_to_wait(&bs->wait, &wait, TASK_UNINTERRUPTIBLE); tag = __bt_get(hctx, bt, last_tag, tags); /* Value set in bt_update_count not visible yet */ bt_update_count(&tags->bitmap_tags, tdepth); /* blk_mq_tag_wakeup_all(tags, false); */ bt = &tags->bitmap_tags; wake_index = atomic_read(&bt->wake_index); ... io_schedule(); ------------------------------------------------------------------------ This patch adds the missing memory barrier. I found this issue when I was looking through the linux source code for places calling waitqueue_active() before wake_up*(), but without preceding memory barriers, after sending a patch to fix a similar issue in drivers/tty/n_tty.c (Details about the original issue can be found here: https://lkml.org/lkml/2015/9/28/849). Signed-off-by: Kosuke Tatsukawa <tatsu@ab.jp.nec.com> Signed-off-by: Jens Axboe <axboe@fb.com>
-rw-r--r--block/blk-mq-tag.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index ed96474d75cb..7a6b6e27fc26 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -75,6 +75,10 @@ void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool include_reserve)
struct blk_mq_bitmap_tags *bt;
int i, wake_index;
+ /*
+ * Make sure all changes prior to this are visible from other CPUs.
+ */
+ smp_mb();
bt = &tags->bitmap_tags;
wake_index = atomic_read(&bt->wake_index);
for (i = 0; i < BT_WAIT_QUEUES; i++) {