[PATCH] btrfs: fix deadlock when writing out space cache

[Josef Bacik <josef@toxicpanda.com>]

From: Josef Bacik <jbacik@fb.com>

If we fail to prepare our pages for whatever reason (out of memory in
our case) we need to make sure to drop the block_group->data_rwsem,
otherwise hilarity ensues.

Signed-off-by: Josef Bacik <jbacik@fb.com>
---
 fs/btrfs/free-space-cache.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index cdc9f4015ec3..a6c643275210 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -1263,8 +1263,12 @@ static int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,
 
 	/* Lock all pages first so we can lock the extent safely. */
 	ret = io_ctl_prepare_pages(io_ctl, inode, 0);
-	if (ret)
+	if (ret) {
+		if (block_group &&
+		    (block_group->flags & BTRFS_BLOCK_GROUP_DATA))
+			up_write(&block_group->data_rwsem);
 		goto out;
+	}
 
 	lock_extent_bits(&BTRFS_I(inode)->io_tree, 0, i_size_read(inode) - 1,
 			 &cached_state);
-- 
2.7.5