Re: [PATCH v4 2/4] btrfs: cleanup btrfs_mount() using btrfs_mount_root()

[David Sterba <dsterba@suse.cz>]

On Fri, Jan 12, 2018 at 06:14:40PM +0800, Anand Jain wrote:
> 
> Misono,
> 
>   This change is causing subsequent (subvol) mount to fail when device
>   option is specified. The simplest eg for failure is ..
>     mkfs.btrfs -qf /dev/sdc /dev/sdb
>     mount -o device=/dev/sdb /dev/sdc /btrfs
>     mount -o device=/dev/sdb /dev/sdc /btrfs1
>        mount: /dev/sdc is already mounted or /btrfs1 busy
> 
>    Looks like
>      blkdev_get_by_path() <-- is failing.
>      btrfs_scan_one_device()
>      btrfs_parse_early_options()
>      btrfs_mount()
> 
>   Which is due to different holders (viz. btrfs_root_fs_type and
>   btrfs_fs_type) one is used for vfs_mount and other for scan,
>   so they form different holders and can't let EXCL open which
>   is needed for both scan and open.

This looks close to what I see in the random test failures. I've
reverted your patch "btrfs: optimize move uuid_mutex closer to the
critical section" as I bisected to it. The uuid mutex around
blkdev_get_path probably protected the concurrent mount and scan so they
did not ask for EXCL at the same time.

Reverting (or removing the patch from the current misc-next) queue is
simpler for me ATM as I want to get to a stable base now, we can add it
later if we understand the issue with the mount/scan.