linux-kernel - [PATCH 3.16.y-ckt 102/216] blk-mq: Avoid that __bt_get

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Message-Id: <1421085933-32536-103-git-send-email-luis.henriques@canonical.com>
Date:	Mon, 12 Jan 2015 18:03:39 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Bart Van Assche <bvanassche@....org>,
	Christoph Hellwig <hch@....de>,
	Robert Elliott <elliott@...com>,
	Ming Lei <ming.lei@...onical.com>,
	Alexander Gordeev <agordeev@...hat.com>,
	Jens Axboe <axboe@...com>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 102/216] blk-mq: Avoid that __bt_get_word() wraps multiple times

3.16.7-ckt4 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Bart Van Assche <bvanassche@....org>

commit 9e98e9d7cf6e9d2ec1cce45e8d5ccaf3f9b386f3 upstream.

If __bt_get_word() is called with last_tag != 0, if the first
find_next_zero_bit() fails, if after wrap-around the
test_and_set_bit() call fails and find_next_zero_bit() succeeds,
if the next test_and_set_bit() call fails and subsequently
find_next_zero_bit() does not find a zero bit, then another
wrap-around will occur. Avoid this by introducing an additional
local variable.

Signed-off-by: Bart Van Assche <bvanassche@....org>
Cc: Christoph Hellwig <hch@....de>
Cc: Robert Elliott <elliott@...com>
Cc: Ming Lei <ming.lei@...onical.com>
Cc: Alexander Gordeev <agordeev@...hat.com>
Signed-off-by: Jens Axboe <axboe@...com>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 block/blk-mq-tag.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 74a4168ea34e..a1bc33974fc4 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -137,6 +137,7 @@ static inline bool hctx_may_queue(struct blk_mq_hw_ctx *hctx,
 static int __bt_get_word(struct blk_align_bitmap *bm, unsigned int last_tag)
 {
 	int tag, org_last_tag, end;
+	bool wrap = last_tag != 0;
 
 	org_last_tag = last_tag;
 	end = bm->depth;
@@ -148,8 +149,9 @@ restart:
 			 * We started with an offset, start from 0 to
 			 * exhaust the map.
 			 */
-			if (org_last_tag && last_tag) {
-				end = last_tag;
+			if (wrap) {
+				wrap = false;
+				end = org_last_tag;
 				last_tag = 0;
 				goto restart;
 			}
-- 
2.1.4

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