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 stable@vger.kernel.org.

thanks,

greg k-h

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

From f2cb2f39ccc30fa13d3ac078d461031a63960e5b Mon Sep 17 00:00:00 2001

From: Filipe Manana fdmanana@suse.com 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.

Fixes: edf064e7c6fec3 ("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

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); }

current->backing_dev_info = inode_to_bdi(inode);