From 8ee1b7b9d8761a7b1a0f37a6e596b5315d27d2f2 Mon Sep 17 00:00:00 2001
From: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>
Date: Fri, 9 Oct 2015 00:35:38 +0000
Subject: [PATCH] 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>
---
block/blk-mq-tag.c | 4 ++++
1 file changed, 4 insertions(+)
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++) {
--
2.34.1