[Ocfs2-devel] [PATCH] fix dead lock caused by ocfs2_defrag_extent

[Larry Chen]

Hi Changwei,

Maybe we need more investigation.

The following is your fix:
lock truncate log inode
     __ocfs2_flush_truncate_log()
	ocfs2_lock_allocators_move_extents()
		unlock truncate log inode

The lock action will happen like following:
lock(truncate_inode)
	lock(sub allocat)
	lock(local_alloc) or lock(global_bitmap)
	
I'm not sure if there is another code path that tries to get the same 
locks but in different order, which may causes dead locks.

Indeed ocfs2 involves too many locks, I would like to reduce the 
deadlock risk at max extent.

Another way is to add an new argument for __ocfs2_flush_truncate which 
indicates whether global bitmap is needed to be locked.


Thanks,
Larry





On 08/30/2018 10:05 AM, Changwei Ge wrote:
> Hi Larry,
> 
> Sorry for this late reply.
> 
> 
> On 2018/8/28 12:46, Larry Chen wrote:
>> Hi Changwei,
>>
>> I think there might be some logic error.
>>
>> Imagine that we remove __ocfs2_flush_truncate_log() in
>> ocfs2_defrag_extent().
>>
>> In ocfs2_reserve_clusters might skip freeing truncate log if there is
>> enough clusters in local. Then __ocfs2_move_extent might return error
>> because ocfs2_truncate_log_append might fail.
> I think changing ocfs2 infrastructure methods like your way is not a
> good idea.
> So how about this way?
> ->lock truncate log inode
> ->__ocfs2_flush_truncate_log()
> ->ocfs2_lock_allocators_move_extents()
> ->unlock truncate log inode
> 
> Thanks
> Changwei
> 
>>
>> Thanks,
>> Larry
>>
>>
>> On 08/28/2018 09:59 AM, Changwei Ge wrote:
>>> Hi Larry,
>>>
>>> I'd like to propose another way to fix this issue, which might be
>>> easier. :-)
>>> Actually, ocfs2_reserve_clusters() inside will flush truncate log if
>>> there is no enough clusters left.
>>> So how about just remove __ocfs2_flush_truncate_log() in
>>> ocfs2_defrag_extent()?
>>>
>>> Thanks,
>>> Changwei
>>>
>>> On 2018/8/27 16:01, Larry Chen wrote:
>>>> ocfs2_defrag_extent may fall into dead lock.
>>>>
>>>> ocfs2_ioctl_move_extents
>>>>      ocfs2_ioctl_move_extents
>>>>        ocfs2_move_extents
>>>>          ocfs2_defrag_extent
>>>>            ocfs2_lock_allocators_move_extents
>>>>
>>>>              ocfs2_reserve_clusters
>>>>                inode_lock GLOBAL_BITMAP_SYSTEM_INODE
>>>>
>>>>        __ocfs2_flush_truncate_log
>>>>                inode_lock GLOBAL_BITMAP_SYSTEM_INODE
>>>>
>>>> As back trace shows above, ocfs2_reserve_clusters will call inode_lock
>>>> against the global bitmap if local allocator has not sufficient
>>>> cluters.
>>>> Once global bitmap could meet the demand, ocfs2_reserve_cluster will
>>>> return success with global bitmap locked.
>>>>
>>>> After ocfs2_reserve_cluster, if truncate log is full,
>>>> __ocfs2_flush_truncate_log will definitely fall into dead lock
>>>> because it needs to inode_lock global bitmap, which has
>>>> already been locked.
>>>>
>>>> To fix this bug, we could remove from
>>>> ocfs2_lock_allocators_move_extents
>>>> the code which intends to lock global allocator, and put the removed
>>>> code after __ocfs2_flush_truncate_log
>>>>
>>>> The ocfs2_lock_allocators_move_extents has been refered by 2 places,
>>>> one
>>>> is here, the other does not need the data allocator context, which
>>>> means
>>>> this patch does not affect the caller so far.
>>>>
>>>>
>>>>
>>>> Signed-off-by: Larry Chen <lchen at suse.com>
>>>> ---
>>>>     fs/ocfs2/move_extents.c | 13 ++++++-------
>>>>     1 file changed, 6 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/fs/ocfs2/move_extents.c b/fs/ocfs2/move_extents.c
>>>> index 7eb3b0a6347e..064dedf40d74 100644
>>>> --- a/fs/ocfs2/move_extents.c
>>>> +++ b/fs/ocfs2/move_extents.c
>>>> @@ -192,13 +192,6 @@ static int
>>>> ocfs2_lock_allocators_move_extents(struct inode *inode,
>>>>             goto out;
>>>>         }
>>>>     -    if (data_ac) {
>>>> -        ret = ocfs2_reserve_clusters(osb, clusters_to_move, data_ac);
>>>> -        if (ret) {
>>>> -            mlog_errno(ret);
>>>> -            goto out;
>>>> -        }
>>>> -    }
>>>>            *credits += ocfs2_calc_extend_credits(osb->sb,
>>>> et->et_root_el);
>>>>     @@ -283,6 +276,12 @@ static int ocfs2_defrag_extent(struct
>>>> ocfs2_move_extents_context *context,
>>>>             }
>>>>         }
>>>>     +    ret = ocfs2_reserve_clusters(osb, *len, &context->data_ac);
>>>> +    if (ret) {
>>>> +        mlog_errno(ret);
>>>> +        goto out_unlock_mutex;
>>>> +    }
>>>> +
>>>>         handle = ocfs2_start_trans(osb, credits);
>>>>         if (IS_ERR(handle)) {
>>>>             ret = PTR_ERR(handle);
>>>
>>
>