Commit f2cb2f39 authored Jun 15, 2020 by Filipe Manana Committed by David Sterba Jun 16, 2020

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.

Fixes: edf064e7 ("btrfs: nowait aio support")
CC: stable@vger.kernel.org # 4.14+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

parent e7a79811

fs/btrfs/file.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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);

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