[Ocfs2-devel] [PATCH] ocfs2: using i_size_read() to access i_size

[Junxiao Bi]

Hi Jeff,

On 07/19/2013 04:30 PM, Jeff Liu wrote:
> On 07/19/2013 04:12 PM, Junxiao Bi wrote:
>
>> Hi Jeff,
>>
>> On 07/19/2013 04:03 PM, Jeff Liu wrote:
>>> Hi Junxiao,
>>>
>>> On 07/19/2013 03:38 PM, Junxiao Bi wrote:
>>>
>>>> Hi Jeff,
>>>>
>>>> On 07/17/2013 07:13 AM, Jeff Liu wrote:
>>>>>>> diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
>>>>>>>>> index 3ce6a8b..7158710 100644
>>>>>>>>> --- a/fs/ocfs2/file.c
>>>>>>>>> +++ b/fs/ocfs2/file.c
>>>>>>>>> @@ -2650,7 +2650,7 @@ static loff_t ocfs2_file_llseek(struct file *file, loff_t offset, int whence)
>>>>> We have the inode mutex around ocfs2_file_llseek(), and that is too extensive
>>>>> to block the concurrent access for particular seek operations.  At least,
>>>>> we can get rid of this lock for SEEK_SET/SEEK_CUR. i.e,  In either case,
>>>>> we can fall through generic_file_llseek() directly without the mutex lock.
>>>> Think about this again, I think we can't remove the mutex, as it is used
>>>> to protect other inode member beside i_size, like
>>>> inode->i_sb->s_maxbytes which is accessed in all SEEK request and also
>>>> more in ocfs2_inode_lock().
>>> Thanks for digging into this.  s_maxbytes is a numeric constant which is
>>> calculated out at OCFS2 mount with filling up the super blocks, IOWs, it
>>> does not need any lock protection IMO.
>>>
>>> Besides that, could you please pointed me out anything am missed?
>> Can inode->i_sb be set to NULL during this?
> No, actually when we get into ocfs2_file_llseek(), the corresponding
> inode should be in VFS inode cache and it should not be a bad inode
> as well, otherwise, it can not go through vfs_llseek(). :)
Thank you for explaining this.
So for SEEK_SET and SEEK_CUR, we may make some improvement, indeed in
gereric_file_llseek(), it also use a spin lock to protect concurrent
SEEK_CUR access.
is there really a perf issue that deserve us to make a hack to hurt code
readability?

Thanks,
Junxiao.
>
> Thanks,
> -Jeff
>
>> Thanks,
>> Junxiao.
>>> Thanks,
>>> -Jeff
>>>
>>>> Thanks,
>>>> Junxiao.
>>>>>>>>>  	case SEEK_SET:
>>>>>>>>>  		break;
>>>>>>>>>  	case SEEK_END:
>>>>>>>>> -		offset += inode->i_size;
>>>>>>>>> +		offset += i_size_read(inode);
>