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

[alex chen]

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.

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.
   ocfs2_rw_lock()
   ocfs2_inode_lock_tracker()
    this function is used to prevent the inode from being modified by another
    nodes in the cluster
 inode_unlock()

> 
> Ben.
>