Re: [PATCH 1/4] Btrfs: fix hang on snapshot creation after RWF_NOWAIT write

[David Sterba <dsterba@suse.cz>]

On Mon, Jun 15, 2020 at 06:46:01PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> 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@vger.kernel.org # 4.13+
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>  fs/btrfs/file.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 2c14312b05e8..04faa04fccd1 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -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);

That's quite ugly that the locking semantics of check_can_nocow is
hidden, this should be cleaned up too.

The whole condition

1909                 if (!(BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
1910                                               BTRFS_INODE_PREALLOC)) ||
1911                     check_can_nocow(BTRFS_I(inode), pos, &count) <= 0)

has 2 parts and it's not obvious from the context when the lock actually is
taken. The flags check could be pushed down to check_can_nocow, the
same but negated condition can be found in btrfs_file_write_iter so this
would make it something like:

	if (check_can_nocow(inode, pos, &count) <= 0) {
		/* fallback */
		return ...;
	}
	/*
	 * the lock is taken and needs to be unlocked at the right time
	 */

Suggestions to rename check_can_nocow welcome too.


>  	}
>  
>  	current->backing_dev_info = inode_to_bdi(inode);
> -- 
> 2.26.2