[3.16.y-ckt stable] Patch "blk-mq: Fix a race between bt_clear_tag() and bt_get()" 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 a race between bt_clear_tag() and bt_get()

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-ckt4.

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 4a99443dfb78124201e4adb31894dd7cca9bf126 Mon Sep 17 00:00:00 2001
From: Bart Van Assche <bvanassche at acm.org>
Date: Tue, 9 Dec 2014 16:58:35 +0100
Subject: blk-mq: Fix a race between bt_clear_tag() and bt_get()

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 at acm.org>
Cc: Christoph Hellwig <hch at lst.de>
Cc: Robert Elliott <elliott at hp.com>
Cc: Ming Lei <ming.lei at canonical.com>
Cc: Alexander Gordeev <agordeev at redhat.com>
Signed-off-by: Jens Axboe <axboe at fb.com>
Signed-off-by: Luis Henriques <luis.henriques at 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