Re: [PATCH] Btrfs: save us a mutex_lock usage when doing quota rescan

[Wang Shilong <wangsl-fnst@cn.fujitsu.com>]

Hello Jan,

> On Tue, May 07, 2013 at 08:15 (+0200), Wang Shilong wrote:
>> If qgroup_rescan worker is in progress, we should ignore
>> the extent that has not been dealt with qgroup_rescan worker,just
>> let them dealt later otherwise we may get wrong qgroup accounting.
>>
>> However, we have checked this before find_all_roots() without spin_lock.
>> When doing qgroup accounting, we don't have to check it again, because
>> during this period,qgroup_rescan worker can deal with more extents and
>> qgroup_rescan_extent->objectid can only go larger, so here the check
>> is unnecessary.
>>
>> Just remove this check, so that we don't need hold qgroup_rescan_lock
>> when doing qgroup accounting.
> 
> NAK.
> 
> After a discussion on that lock the last thing in this thread I see is ...
> 
> On Wed, May 01, 2013 at 13:57 (+0200), Jan Schmidt wrote:
>> Now I see what you mean. The second check is only required when we start
>> a rescan operation after the initial check in btrfs_qgroup_account_ref.
> 
> Please continue on that argument, your commit message doesn't explain at all why
> we should be safe to remove this check.


You are right! 

Thanks,
Wang

> 
> -Jan
> 
>> Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
>> ---
>>  fs/btrfs/qgroup.c |    9 ---------
>>  1 files changed, 0 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
>> index d059d86..2710784 100644
>> --- a/fs/btrfs/qgroup.c
>> +++ b/fs/btrfs/qgroup.c
>> @@ -1445,15 +1445,7 @@ int btrfs_qgroup_account_ref(struct btrfs_trans_handle *trans,
>>  	if (ret < 0)
>>  		return ret;
>>  
>> -	mutex_lock(&fs_info->qgroup_rescan_lock);
>>  	spin_lock(&fs_info->qgroup_lock);
>> -	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) {
>> -		if (fs_info->qgroup_rescan_progress.objectid <= node->bytenr) {
>> -			ret = 0;
>> -			goto unlock;
>> -		}
>> -	}
>> -
>>  	quota_root = fs_info->quota_root;
>>  	if (!quota_root)
>>  		goto unlock;
>> @@ -1492,7 +1484,6 @@ int btrfs_qgroup_account_ref(struct btrfs_trans_handle *trans,
>>  
>>  unlock:
>>  	spin_unlock(&fs_info->qgroup_lock);
>> -	mutex_unlock(&fs_info->qgroup_rescan_lock);
>>  	ulist_free(roots);
>>  
>>  	return ret;
>>
> 
>