[PATCH 3.11 201/208] dm array: fix a reference counting bug in shadow_ablock

[Luis Henriques]

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

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

From: Joe Thornber <ejt@xxxxxxxxxx>

commit ed9571f0cf1fe09d3506302610f3ccdfa1d22c4a upstream.

An old array block could have its reference count decremented below
zero when it is being replaced in the btree by a new array block.

The fix is to increment the old ablock's reference count just before
inserting a new ablock into the btree.

Signed-off-by: Joe Thornber <ejt@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 drivers/md/persistent-data/dm-array.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/md/persistent-data/dm-array.c b/drivers/md/persistent-data/dm-array.c
index af96e24..1d75b1d 100644
--- a/drivers/md/persistent-data/dm-array.c
+++ b/drivers/md/persistent-data/dm-array.c
@@ -317,8 +317,16 @@ static int shadow_ablock(struct dm_array_info *info, dm_block_t *root,
 	 * The shadow op will often be a noop.  Only insert if it really
 	 * copied data.
 	 */
-	if (dm_block_location(*block) != b)
+	if (dm_block_location(*block) != b) {
+		/*
+		 * dm_tm_shadow_block will have already decremented the old
+		 * block, but it is still referenced by the btree.  We
+		 * increment to stop the insert decrementing it below zero
+		 * when overwriting the old value.
+		 */
+		dm_tm_inc(info->btree_info.tm, b);
 		r = insert_ablock(info, index, *block, root);
+	}
 
 	return r;
 }
-- 
1.8.3.2

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