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

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Aug 27, 2019 at 10:28:22AM +0800, kaixuxia wrote:
> 
> 
> On 2019/8/27 10:13, Darrick J. Wong wrote:
> > On Tue, Aug 27, 2019 at 10:07:43AM +0800, kaixuxia wrote:
> >> 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.
> > 
> > Ok.  By the way, do you have a quick reproducer that we can put into
> > xfstests?
> 
> Yeah, I have a quick reproducer that mkfs.xfs the disk with agcount=1
> or agcount=2, and I can send the testcase to xfstests.
> 
> In a word, I will send two patches to fix the different deadlock
> problems, and then the corresponding quick reproducers will also
> be sent later.

Er... did you send the second patch to fix the droplink deadlock?  Or
the reproducers?

--D

> > --D
> > 
> >>
> >>>
> >>> --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
> 
> -- 
> kaixuxia