Re: [PATCH 4.4 13/16] ocfs2: should wait dio before inode lock in ocfs2_setattr()

[alex chen]

On 2017/12/8 13:36, Ben Hutchings wrote:
> On Fri, 2017-12-08 at 12:03 +0800, alex chen wrote:
>>
>> On 2017/12/8 10:26, Ben Hutchings wrote:
>>> On Fri, 2017-12-08 at 08:39 +0800, alex chen wrote:
>>>>
>>>> On 2017/12/8 2:25, Ben Hutchings wrote:
>>>>> On Wed, 2017-12-06 at 09:02 +0800, alex chen wrote:
>>>>>> Hi Ben,
>>>>>>
>>>>>> Thanks for your reply.
>>>>>>
>>>>>> On 2017/12/5 23:49, Ben Hutchings wrote:
>>>>>>> On Wed, 2017-11-22 at 11:12 +0100, Greg Kroah-Hartman wrote:
>>>>>>>> 4.4-stable review patch.  If anyone has any objections,
>>>>>>>> please let me know.
>>>>>>>>
>>>>>>>> ------------------
>>>>>>>>
>>>>>>>> From: alex chen <alex.chen@xxxxxxxxxx>
>>>>>>>>
>>>>>>>> commit 28f5a8a7c033cbf3e32277f4cc9c6afd74f05300 upstream.
>>>>>>>>
>>>>>>>> we should wait dio requests to finish before inode lock in
>>>>>>>> ocfs2_setattr(), otherwise the following deadlock will
>>>>>>>> happen:
>>>>>>>
>>>>>>> [...]
>>>>>>>
>>>>>>> I looked at the kernel-doc for inode_dio_wait():
>>>>>>>
>>>>>>> /**
>>>>>>>  * inode_dio_wait - wait for outstanding DIO requests to finish
>>>>>>>  * @inode: inode to wait for
>>>>>>>  *
>>>>>>>  * Waits for all pending direct I/O requests to finish so that we can
>>>>>>>  * proceed with a truncate or equivalent operation.
>>>>>>>  *
>>>>>>>  * Must be called under a lock that serializes taking new references
>>>>>>>  * to i_dio_count, usually by inode->i_mutex.
>>>>>>>  */
>>>>>>>
>>>>>>> Now that ocfs2_setattr() calls this outside of the inode locked region,
>>>>>>> what prevents another task adding a new dio request immediately
>>>>>>> afterward?
>>>>>>>
>>>>>>
>>>>>> In the kernel 4.6, firstly, we use the inode_lock() in do_truncate() to
>>>>>> prevent another bio to be issued from this node.
>>>>>
>>>>> [...]
>>>>>
>>>>> Yes but there seems to be a race condition - after the call to
>>>>> inode_dio_wait() and before the call to inode_lock(), another dio
>>>>> request can be added.
>>>
>>> Sorry, I've been mixing up inode_lock() and ocfs2_inode_lock(). 
>>> However:
>>>
>>>> In the truncating file situation, the lock order is as follow:
>>>> do_truncate()
>>>>  inode_lock()
>>>>  notify_change()
>>>>   ocfs2_setattr()
>>>>    inode_dio_wait()
>>>>     --here it is under the protect of inode_lock(), so another dio requests
>>>>       from another process will not be added.
>>>
>>> only DIO reads seem to take the inode lock.
>>>
>>
>> I do not clearly understand what you mean.
>> The inode_lock() will be called in ocfs2_file_write_iter().
> 
> Oh I see.  I didn't realise that was part of the call chain.
> 
>> You mean only DIO writes seem to take the inode_lock()?
> 
> I did mean reads, as do_blockdev_direct_IO() may call inode_lock() for
> reads - but ocfs2 doesn't set the flag for that.  Maybe that's OK?

I think you are right, we should set the DIO_LOCKING flag in ocfs2_direct_IO().

Thanks,
Alex
> 
>> BTW, in this patch, I just adjusted the inode_dio_wait() to the front of the ocfs2_rw_lock()
>> and didn't adjust the order of inode_lock() and inode_dio_wait().
> 
> Right.  I think you've convinced me to stop worrying about this.
> 
> Ben.
>