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

[kaixuxia <xiakaixu1987@gmail.com>]

On 2019/8/23 22:07, Brian Foster wrote:
> On Fri, Aug 23, 2019 at 12:56:53PM +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.
>>
> ...
>>
>> 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>
>> ---
>>  fs/xfs/xfs_inode.c | 85 +++++++++++++++++++++++++++---------------------------
>>  1 file changed, 43 insertions(+), 42 deletions(-)
>>
>> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
>> index 6467d5e..584b9d1 100644
>> --- a/fs/xfs/xfs_inode.c
>> +++ b/fs/xfs/xfs_inode.c
>> @@ -3282,9 +3282,10 @@ 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) {
>> +	if (!target_ip) {
> 
> Not sure there's really a point to this change now.
> 
>>  		/*
>>  		 * If there's no space reservation, check the entry will
>>  		 * fit before actually inserting it.
>> @@ -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))) {
> 
> ^ This line needs one more space of indent because it's encapsulated by
> the opening brace one line up. The braces around xfs_dir_isempty() also
> look spurious, FWIW. With those nits fixed, the rest looks good to me:
> 
> Reviewed-by: Brian Foster <bfoster@redhat.com>

Thanks! I will send the new patch to address them with your 
Reviewed-by.
> 
> Thanks for the patch.
> 
> Brian
> 
>> +			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
>> @@ -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