Re: [PATCH RESEND v4 2/8] btrfs: Make btrfs_parse_options() parse mount option in a atomic way

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

-------- Original Message --------
Subject: Re: [PATCH RESEND v4 2/8] btrfs: Make btrfs_parse_options() 
parse mount option in a atomic way
From: Miao Xie <miaoxie@huawei.com>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>, <linux-btrfs@vger.kernel.org>
Date: 2015年01月30日 09:29
> On Fri, 30 Jan 2015 09:20:46 +0800, Qu Wenruo wrote:
>>> Here need ACCESS_ONCE to wrap info->mount_opt, or the complier might use
>>> info->mount_opt instead of new_opt.
>> Thanks for pointing out this one.
>>> But I worried that this is not key reason of the wrong space cache. Could
>>> you explain the race condition which caused the wrong space cache?
>>>
>>> Thanks
>>> Miao
>> CPU0:
>> remount()
>> |- sync_fs() <- after sync_fs() we can start new trans
>> |- btrfs_parse_options()     CPU1:
>>                      |- start_transaction()
>>                      |- Do some bg allocation, not recorded in space_cache.
> I think it is a bug if a free space is not recorded in space cache. Could you
> explain why it is not recorded?
>
> Thanks
> Miao
IIRC, in that window, the fs_info->mount_opt's SPACE_CACHE bit is cleared.
So space cache is not recorded.

Thanks,
Qu
>
>>       |- set SPACE_CACHE bit due to cache_gen
>>
>>                      |- commit_transaction()
>>                          |- write space cache and update cache_gen.
>>                              but since some of it is not recorded in space cache,
>>                              the space cache missing some records.
>>       |- clear SPACE_CACHE bit dut to nospace_cache
>>
>> So the space cache is wrong.
>>
>> Thanks,
>> Qu
>>>> +    }
>>>>        kfree(orig);
>>>>        return ret;
>>>>    }
>>>>
>> .
>>