Re: [PATCH v2] xfs: Fix ABBA deadlock between AGI and AGF in rename()

[kaixuxia <xiakaixu1987@gmail.com>]

On 2019/8/27 8:41, Darrick J. Wong wrote:
> On Sat, Aug 24, 2019 at 11:45:15AM +0800, kaixuxia wrote:
>> When performing rename operation with RENAME_WHITEOUT flag, we will
>> hold AGF lock to allocate or free extents in manipulating the dirents
>> firstly, and then doing the xfs_iunlink_remove() call last to hold
>> AGI lock to modify the tmpfile info, so we the lock order AGI->AGF.
>>
>> The big problem here is that we have an ordering constraint on AGF
>> and AGI locking - inode allocation locks the AGI, then can allocate
>> a new extent for new inodes, locking the AGF after the AGI. Hence
>> the ordering that is imposed by other parts of the code is AGI before
>> AGF. So we get an ABBA deadlock between the AGI and AGF here.
>>
>> Process A:
>> Call trace:
>>  ? __schedule+0x2bd/0x620
>>  schedule+0x33/0x90
>>  schedule_timeout+0x17d/0x290
>>  __down_common+0xef/0x125
>>  ? xfs_buf_find+0x215/0x6c0 [xfs]
>>  down+0x3b/0x50
>>  xfs_buf_lock+0x34/0xf0 [xfs]
>>  xfs_buf_find+0x215/0x6c0 [xfs]
>>  xfs_buf_get_map+0x37/0x230 [xfs]
>>  xfs_buf_read_map+0x29/0x190 [xfs]
>>  xfs_trans_read_buf_map+0x13d/0x520 [xfs]
>>  xfs_read_agf+0xa6/0x180 [xfs]
>>  ? schedule_timeout+0x17d/0x290
>>  xfs_alloc_read_agf+0x52/0x1f0 [xfs]
>>  xfs_alloc_fix_freelist+0x432/0x590 [xfs]
>>  ? down+0x3b/0x50
>>  ? xfs_buf_lock+0x34/0xf0 [xfs]
>>  ? xfs_buf_find+0x215/0x6c0 [xfs]
>>  xfs_alloc_vextent+0x301/0x6c0 [xfs]
>>  xfs_ialloc_ag_alloc+0x182/0x700 [xfs]
>>  ? _xfs_trans_bjoin+0x72/0xf0 [xfs]
>>  xfs_dialloc+0x116/0x290 [xfs]
>>  xfs_ialloc+0x6d/0x5e0 [xfs]
>>  ? xfs_log_reserve+0x165/0x280 [xfs]
>>  xfs_dir_ialloc+0x8c/0x240 [xfs]
>>  xfs_create+0x35a/0x610 [xfs]
>>  xfs_generic_create+0x1f1/0x2f0 [xfs]
>>  ...
>>
>> Process B:
>> Call trace:
>>  ? __schedule+0x2bd/0x620
>>  ? xfs_bmapi_allocate+0x245/0x380 [xfs]
>>  schedule+0x33/0x90
>>  schedule_timeout+0x17d/0x290
>>  ? xfs_buf_find+0x1fd/0x6c0 [xfs]
>>  __down_common+0xef/0x125
>>  ? xfs_buf_get_map+0x37/0x230 [xfs]
>>  ? xfs_buf_find+0x215/0x6c0 [xfs]
>>  down+0x3b/0x50
>>  xfs_buf_lock+0x34/0xf0 [xfs]
>>  xfs_buf_find+0x215/0x6c0 [xfs]
>>  xfs_buf_get_map+0x37/0x230 [xfs]
>>  xfs_buf_read_map+0x29/0x190 [xfs]
>>  xfs_trans_read_buf_map+0x13d/0x520 [xfs]
>>  xfs_read_agi+0xa8/0x160 [xfs]
>>  xfs_iunlink_remove+0x6f/0x2a0 [xfs]
>>  ? current_time+0x46/0x80
>>  ? xfs_trans_ichgtime+0x39/0xb0 [xfs]
>>  xfs_rename+0x57a/0xae0 [xfs]
>>  xfs_vn_rename+0xe4/0x150 [xfs]
>>  ...
>>
>> In this patch we move the xfs_iunlink_remove() call to
>> before acquiring the AGF lock to preserve correct AGI/AGF locking
>> order.
>>
>> Signed-off-by: kaixuxia <kaixuxia@tencent.com>
>> Reviewed-by: Brian Foster <bfoster@redhat.com>
>> ---
>>  fs/xfs/xfs_inode.c | 83 +++++++++++++++++++++++++++---------------------------
>>  1 file changed, 42 insertions(+), 41 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
>> index 6467d5e..8ffd44f 100644
>> --- a/fs/xfs/xfs_inode.c
>> +++ b/fs/xfs/xfs_inode.c
>> @@ -3282,7 +3282,8 @@ struct xfs_iunlink {
>>  					spaceres);
>>  
>>  	/*
>> -	 * Set up the target.
>> +	 * Check for expected errors before we dirty the transaction
>> +	 * so we can return an error without a transaction abort.
>>  	 */
>>  	if (target_ip == NULL) {
>>  		/*
>> @@ -3294,6 +3295,46 @@ struct xfs_iunlink {
>>  			if (error)
>>  				goto out_trans_cancel;
>>  		}
>> +	} else {
>> +		/*
>> +		 * If target exists and it's a directory, check that whether
>> +		 * it can be destroyed.
>> +		 */
>> +		if (S_ISDIR(VFS_I(target_ip)->i_mode) &&
>> +		    (!xfs_dir_isempty(target_ip) ||
>> +		     (VFS_I(target_ip)->i_nlink > 2))) {
>> +			error = -EEXIST;
>> +			goto out_trans_cancel;
>> +		}
>> +	}
>> +
>> +	/*
>> +	 * Directory entry creation below may acquire the AGF. Remove
>> +	 * the whiteout from the unlinked list first to preserve correct
>> +	 * AGI/AGF locking order. This dirties the transaction so failures
>> +	 * after this point will abort and log recovery will clean up the
>> +	 * mess.
>> +	 *
>> +	 * For whiteouts, we need to bump the link count on the whiteout
>> +	 * inode. After this point, we have a real link, clear the tmpfile
>> +	 * state flag from the inode so it doesn't accidentally get misused
>> +	 * in future.
>> +	 */
>> +	if (wip) {
>> +		ASSERT(VFS_I(wip)->i_nlink == 0);
>> +		error = xfs_iunlink_remove(tp, wip);
>> +		if (error)
>> +			goto out_trans_cancel;
>> +
>> +		xfs_bumplink(tp, wip);
>> +		xfs_trans_log_inode(tp, wip, XFS_ILOG_CORE);
>> +		VFS_I(wip)->i_state &= ~I_LINKABLE;
>> +	}
>> +
>> +	/*
>> +	 * Set up the target.
>> +	 */
>> +	if (target_ip == NULL) {
>>  		/*
>>  		 * If target does not exist and the rename crosses
>>  		 * directories, adjust the target directory link count
>> @@ -3312,22 +3353,6 @@ struct xfs_iunlink {
>>  		}
>>  	} else { /* target_ip != NULL */
>>  		/*
>> -		 * If target exists and it's a directory, check that both
>> -		 * target and source are directories and that target can be
>> -		 * destroyed, or that neither is a directory.
>> -		 */
>> -		if (S_ISDIR(VFS_I(target_ip)->i_mode)) {
>> -			/*
>> -			 * Make sure target dir is empty.
>> -			 */
>> -			if (!(xfs_dir_isempty(target_ip)) ||
>> -			    (VFS_I(target_ip)->i_nlink > 2)) {
>> -				error = -EEXIST;
>> -				goto out_trans_cancel;
>> -			}
>> -		}
>> -
>> -		/*
>>  		 * Link the source inode under the target name.
>>  		 * If the source inode is a directory and we are moving
>>  		 * it across directories, its ".." entry will be
> 
> ...will be replaced and then we droplink the target_ip.
> 
> Question: Will we have the same ABBA deadlock potential here if we have
> to allocate a block from AG 2 to hold the directory entry, but then we
> drop target_ip onto the unlinked list, and target_ip was also from AG 2?
> We also have to lock the AGI to put things on the unlinked list.
> 
> Granted, that's a slightly different use case, but they seem related...

Right, we will have the ABBA deadlock here if we have to allocate the
block fist and then put the target_ip on the unlinked list. Of course,
we need to fix it, but these two deadlock problems have different use
case and different reasons, maybe it's better that we fix them with
different patches, and then the subject of this patch need to be
changed...
I also can send another patch to fix the new deadlock problem.

> 
> --D
> 
>> @@ -3417,30 +3442,6 @@ struct xfs_iunlink {
>>  	if (error)
>>  		goto out_trans_cancel;
>>  
>> -	/*
>> -	 * For whiteouts, we need to bump the link count on the whiteout inode.
>> -	 * This means that failures all the way up to this point leave the inode
>> -	 * on the unlinked list and so cleanup is a simple matter of dropping
>> -	 * the remaining reference to it. If we fail here after bumping the link
>> -	 * count, we're shutting down the filesystem so we'll never see the
>> -	 * intermediate state on disk.
>> -	 */
>> -	if (wip) {
>> -		ASSERT(VFS_I(wip)->i_nlink == 0);
>> -		xfs_bumplink(tp, wip);
>> -		error = xfs_iunlink_remove(tp, wip);
>> -		if (error)
>> -			goto out_trans_cancel;
>> -		xfs_trans_log_inode(tp, wip, XFS_ILOG_CORE);
>> -
>> -		/*
>> -		 * Now we have a real link, clear the "I'm a tmpfile" state
>> -		 * flag from the inode so it doesn't accidentally get misused in
>> -		 * future.
>> -		 */
>> -		VFS_I(wip)->i_state &= ~I_LINKABLE;
>> -	}
>> -
>>  	xfs_trans_ichgtime(tp, src_dp, XFS_ICHGTIME_MOD | XFS_ICHGTIME_CHG);
>>  	xfs_trans_log_inode(tp, src_dp, XFS_ILOG_CORE);
>>  	if (new_parent)
>> -- 
>> 1.8.3.1
>>
>> -- 
>> kaixuxia

-- 
kaixuxia