Questions about qgroup_lock and qgroup_ioctl_lock in enable/disable quota

[Sidong Yang <realwakka@gmail.com>]

Hi. 

As a newbie, I have some questions about qgroup_ioctl_lock and
qgroup_lock in enable/disable quota.

I'm reading btrfs_quota_enable() in qgroup.c, I'm confused about
qgroup_ioctl_lock. It seems that in the begining of the function, it
checkes fs_info->quota_root with qgroup_ioctl_lock locked. And I feel
like that fs_info->qgroup_ioctl_lock protects fs_info->quota_root. After
that it releases qgroup_ioctl_lock and starts transaction in line 1005.

I understand that there was problem about deadlock between
qgroup_ioctl_lock and the vfs freeze semaphores. And it also lock
qgroup_ioctl_locki again. Just after that, There is code checkes
fs_info->quota_root like below. 

lock qgroup_ioctl_lock => check fs_info->quota_root => unlock
qgroup_ioctl_lock => unlock qgroup_ioctl_lock => start transaction =>
check fs_info->quota_root

I think there is no need to double checks. Is it right?

And the end of the function, It seems that the code as below.

unlock qgroup_ioctl_lock => commit transaction => lock qgroup_ioctl_lock
=> lock qgroup_lock => set fs_info->quota_root => unlock qgroup_lock =>
unlock qgroup_ioctl_lock

For the beginning of the function, there was no qgroup_lock for check
fs_info->quota_root. It should be protected?

I think the typical pattern below works and it's simple for the function.

start transaction => lock qgroup_ioctl_lock => unlock qgroup_ioctl_lock
=> commit transaction

Will there be any problems with this implementation?