[3.16.y-ckt stable] Patch "blk-mq: fix potential hang if rolling wakeup depth is too high" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    blk-mq: fix potential hang if rolling wakeup depth is too high

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 53c99bcae873f869212c92718b08cbfb1f9cf49b Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe at fb.com>
Date: Tue, 7 Oct 2014 08:39:20 -0600
Subject: blk-mq: fix potential hang if rolling wakeup depth is too high

commit abab13b5c4fd1fec4f9a61622548012d93dc2831 upstream.

We currently divide the queue depth by 4 as our batch wakeup
count, but we split the wakeups over BT_WAIT_QUEUES number of
wait queues. This defaults to 8. If the product of the resulting
batch wake count and BT_WAIT_QUEUES is higher than the device
queue depth, we can get into a situation where a task goes to
sleep waiting for a request, but never gets woken up.

Reported-by: Bart Van Assche <bvanassche at acm.org>
Fixes: 4bb659b156996
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 block/blk-mq-tag.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index c1b92426c95e..74a4168ea34e 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -463,8 +463,8 @@ static void bt_update_count(struct blk_mq_bitmap_tags *bt,
 	}

 	bt->wake_cnt = BT_WAIT_BATCH;
-	if (bt->wake_cnt > depth / 4)
-		bt->wake_cnt = max(1U, depth / 4);
+	if (bt->wake_cnt > depth / BT_WAIT_QUEUES)
+		bt->wake_cnt = max(1U, depth / BT_WAIT_QUEUES);

 	bt->depth = depth;
 }
--
2.1.0