Re: [Ocfs2-devel] [PATCH V3] ocfs2: fix dead lock caused by ocfs2_defrag_extent

[Changwei Ge]

Hello Joseph,

On 2018/11/1 17:01, Joseph Qi wrote:
> Hi Larry,
> 
> On 18/11/1 15:14, Larry Chen wrote:
>> ocfs2_defrag_extent may fall into deadlock.
>>
>> 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 backtrace 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 deadlock 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().
>>
>> ocfs2_lock_allocators_move_extents() is referred 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.
>>
>> Change log:
>> 1. Correct the function comment.
>> 2. Remove unused argument from ocfs2_lock_meta_allocator_move_extents.
>>
>> Signed-off-by: Larry Chen <lchen@xxxxxxxx>
>> ---
>>   fs/ocfs2/move_extents.c | 47 ++++++++++++++++++++++++++---------------------
>>   1 file changed, 26 insertions(+), 21 deletions(-)
>>

> IMO, here clusters_to_move is only for data_ac, since we change this
> function to only handle meta_ac, I'm afraid clusters_to_move related
> logic has to been changed correspondingly.

I think we can't remove *clusters_to_move* from here as clusters can be reserved latter outsides this function, but we
still have to reserve metadata(extents) in advance.
So we need that argument.

Thanks,
Changwei

> 
> Thanks,
> Joseph
>>   					u32 extents_to_split,
>>   					struct ocfs2_alloc_context **meta_ac,
>> -					struct ocfs2_alloc_context **data_ac,
>>   					int extra_blocks,
>>   					int *credits)
>>   {
>> @@ -192,13 +188,6 @@ 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);
>>   
>> @@ -257,10 +246,10 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
>>   		}
>>   	}
>>   
>> -	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, *len, 1,
>> -						 &context->meta_ac,
>> -						 &context->data_ac,
>> -						 extra_blocks, &credits);
>> +	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
>> +						*len, 1,
>> +						&context->meta_ac,
>> +						extra_blocks, &credits);
>>   	if (ret) {
>>   		mlog_errno(ret);
>>   		goto out;
>> @@ -283,6 +272,21 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
>>   		}
>>   	}
>>   
>> +	/*
>> +	 * Make sure ocfs2_reserve_cluster is called after
>> +	 * __ocfs2_flush_truncate_log, otherwise, dead lock may happen.
>> +	 *
>> +	 * If ocfs2_reserve_cluster is called
>> +	 * before __ocfs2_flush_truncate_log, dead lock on global bitmap
>> +	 * may happen.
>> +	 *
>> +	 */
>> +	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);
>> @@ -600,9 +604,10 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
>>   		}
>>   	}
>>   
>> -	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, len, 1,
>> -						 &context->meta_ac,
>> -						 NULL, extra_blocks, &credits);
>> +	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
>> +						len, 1,
>> +						&context->meta_ac,
>> +						extra_blocks, &credits);
>>   	if (ret) {
>>   		mlog_errno(ret);
>>   		goto out;
>>
> 
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