From f2cb2f39ccc30fa13d3ac078d461031a63960e5b Mon Sep 17 00:00:00 2001 From: Filipe Manana 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 Reviewed-by: David Sterba Signed-off-by: David Sterba --- fs/btrfs/file.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index 2c14312..04faa04 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); -- 2.7.4