[PATCH 3.16.y-ckt 034/128] dm thin metadata: fix bug when taking a metadata snapshot

[Luis Henriques]

3.16.7-ckt23 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Joe Thornber <ejt@xxxxxxxxxx>

commit 49e99fc717f624aa75ca755d6e7bc029efd3f0e9 upstream.

When you take a metadata snapshot the btree roots for the mapping and
details tree need to have their reference counts incremented so they
persist for the lifetime of the metadata snap.

The roots being incremented were those currently written in the
superblock, which could possibly be out of date if concurrent IO is
triggering new mappings, breaking of sharing, etc.

Fix this by performing a commit with the metadata lock held while taking
a metadata snapshot.

Signed-off-by: Joe Thornber <ejt@xxxxxxxxxx>
Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 drivers/md/dm-thin-metadata.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/md/dm-thin-metadata.c b/drivers/md/dm-thin-metadata.c
index 3412b86e79fd..7768de60f699 100644
--- a/drivers/md/dm-thin-metadata.c
+++ b/drivers/md/dm-thin-metadata.c
@@ -1205,6 +1205,12 @@ static int __reserve_metadata_snap(struct dm_pool_metadata *pmd)
 	dm_block_t held_root;
 
 	/*
+	 * We commit to ensure the btree roots which we increment in a
+	 * moment are up to date.
+	 */
+	__commit_transaction(pmd);
+
+	/*
 	 * Copy the superblock.
 	 */
 	dm_sm_inc_block(pmd->metadata_sm, THIN_SUPERBLOCK_LOCATION);