On Wed, Apr 27, 2016 at 4:14 PM, David Sterba <dsterba@xxxxxxxx> wrote:
> A user reported mount failures with EBUSY during boot, there's root
> partition and many subvolumes, mounted via /etc/fstab.
>
> The failure depends on timing, when multiple subvolumes reach the code
> between superblock creation in RO mode, while the subvolumes are RW.
> This discrepancy leads to EBUSY and the code has been there since ages.
>
> If the subvolumes are mounted after a short delay, there's no EBUSY.
> There's no missing locking, the supreblock creation is atomic and the
> error code seems to be just artificial. We support different RO/RW
> mounts in mount_subvol and do the relevant adjustments if the flags do
> not match.
>
> There are no apparent problems if the check and EBUSY are dropped, no
> wrong refcounting, umount works etc.
>
> Reproducer:
>
> mkdir -p mnt $(eval echo mnt{1..10})
> mkfs.btrfs -f /dev/sdx
> mount /dev/sdx mnt
> for i in `seq 10`; do btrfs subvol create subv$i; done
> loops=0
> while [ $loops -lt 10 ]; do
> umount mnt*
> mount -o ro /dev/sdx mnt &
> for i in `seq 10`; do
> mount -o subvol=subv$i /dev/sdx mnt$i || \
> echo "FAIL: mnt$i, loop $loops" > log-$i-$loops &
> fi
> done
> wait
> : $((loops++))
> done
>
> Do 10 iterations, try to run subvolume mounts concurrently, log mount
> failures to files, the actual EBUSY errors are on screen.
This is easy to reproduce, can you please make a test case for xfstests?
Thanks.
>
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Sterba <dsterba@xxxxxxxx>
> ---
> fs/btrfs/super.c | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 00b8f37cc306..0c398643d2a1 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1588,8 +1588,6 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
> if (s->s_root) {
> btrfs_close_devices(fs_devices);
> free_fs_info(fs_info);
> - if ((flags ^ s->s_flags) & MS_RDONLY)
> - error = -EBUSY;
> } else {
> snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
> btrfs_sb(s)->bdev_holder = fs_type;
> --
> 2.7.1
>
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