[PATCH] btrfs: do not return EBUSY on concurrent subvolume mounts

[David Sterba <dsterba@suse.com>]

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.

CC: stable@vger.kernel.org
Signed-off-by: David Sterba <dsterba@suse.com>
---
 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