[PATCH 5.7 225/265] btrfs: fix hang on snapshot creation after RWF_NOWAIT write

[Sasha Levin]

From: Filipe Manana <fdmanana@xxxxxxxx>

commit f2cb2f39ccc30fa13d3ac078d461031a63960e5b upstream.

If we do a successful RWF_NOWAIT write we end up locking the snapshot lock
of the inode, through a call to check_can_nocow(), but we never unlock it.

This means the next attempt to create a snapshot on the subvolume will
hang forever.

Trivial reproducer:

  $ mkfs.btrfs -f /dev/sdb
  $ mount /dev/sdb /mnt

  $ touch /mnt/foobar
  $ chattr +C /mnt/foobar
  $ xfs_io -d -c "pwrite -S 0xab 0 64K" /mnt/foobar
  $ xfs_io -d -c "pwrite -N -V 1 -S 0xfe 0 64K" /mnt/foobar

  $ btrfs subvolume snapshot -r /mnt /mnt/snap
    --> hangs

Fix this by unlocking the snapshot lock if check_can_nocow() returned
success.

Fixes: edf064e7c6fec3 ("btrfs: nowait aio support")
CC: stable@xxxxxxxxxxxxxxx # 4.14+
Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/btrfs/file.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 719e68ab552c5..484803f8b2290 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1922,6 +1922,8 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 			inode_unlock(inode);
 			return -EAGAIN;
 		}
+		/* check_can_nocow() locks the snapshot lock on success */
+		btrfs_drew_write_unlock(&root->snapshot_lock);
 	}
 
 	current->backing_dev_info = inode_to_bdi(inode);
-- 
2.25.1