FAILED: patch "[PATCH] btrfs: fix hang on snapshot creation after RWF_NOWAIT write" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f2cb2f39ccc30fa13d3ac078d461031a63960e5b Mon Sep 17 00:00:00 2001
From: Filipe Manana <[email protected]>
Date: Mon, 15 Jun 2020 18:46:01 +0100
Subject: [PATCH] btrfs: fix hang on snapshot creation after RWF_NOWAIT write

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.

index 2c14312b05e8..04faa04fccd1 100644
@@ -1914,6 +1914,8 @@ 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);