On Mon, Jun 15, 2020 at 06:46:01PM +0100, fdmanana@xxxxxxxxxx wrote:
> From: Filipe Manana <fdmanana@xxxxxxxx>
>
> 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 # 4.13+
> Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
> ---
> 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
