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

[Miao Xie <miaoxie@huawei.com>]

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

>      |- 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;
>>>   }
>>>
> 
> .
>