[PATCH 3.16 192/346] bcache: register_bcache(): call blkdev_put() when cache_alloc() fails

[Ben Hutchings]

3.16.39-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Eric Wheeler <git@xxxxxxxxxxxxxxxxxx>

commit d9dc1702b297ec4a6bb9c0326a70641b322ba886 upstream.

register_cache() is supposed to return an error string on error so that
register_bcache() will will blkdev_put and cleanup other user counters,
but it does not set 'char *err' when cache_alloc() fails (eg, due to
memory pressure) and thus register_bcache() performs no cleanup.

register_bcache() <----------\  <- no jump to err_close, no blkdev_put()
   |                         |
   +->register_cache()       |  <- fails to set char *err
         |                   |
         +->cache_alloc() ---/  <- returns error

This patch sets `char *err` for this failure case so that register_cache()
will cause register_bcache() to correctly jump to err_close and do
cleanup.  This was tested under OOM conditions that triggered the bug.

Signed-off-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/md/bcache/super.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1854,7 +1854,7 @@ static int register_cache(struct cache_s
 				  struct block_device *bdev, struct cache *ca)
 {
 	char name[BDEVNAME_SIZE];
-	const char *err = NULL;
+	const char *err = NULL; /* must be set for any error case */
 	int ret = 0;
 
 	memcpy(&ca->sb, sb, sizeof(struct cache_sb));
@@ -1871,8 +1871,13 @@ static int register_cache(struct cache_s
 		ca->discard = CACHE_DISCARD(&ca->sb);
 
 	ret = cache_alloc(sb, ca);
-	if (ret != 0)
+	if (ret != 0) {
+		if (ret == -ENOMEM)
+			err = "cache_alloc(): -ENOMEM";
+		else
+			err = "cache_alloc(): unknown error";
 		goto err;
+	}
 
 	if (kobject_add(&ca->kobj, &part_to_dev(bdev->bd_part)->kobj, "bcache")) {
 		err = "error calling kobject_add";