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

[kaixuxia <xiakaixu1987@gmail.com>]

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
@@ -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