[PATCH v3] xfs: Fix agi&agf ABBA deadlock when performing rename with RENAME_WHITEOUT flag

[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 the ABBA agi&agf deadlock 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>
---
  fs/xfs/xfs_inode.c | 61 ++++++++++++++++++++++++++++++++++--------------------
  1 file changed, 38 insertions(+), 23 deletions(-)

diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
index 6467d5e..cf06568 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.
+	 * Error checks before we dirty the transaction, return
+	 * the error code if check failed and the filesystem is clean.
  	 */
  	if (target_ip == NULL) {
  		/*
@@ -3294,6 +3295,40 @@ struct xfs_iunlink {
  			if (error)
  				goto out_trans_cancel;
  		}
+	} else {
+		/*
+		 * 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;
+			}
+		}
+	}
+
+	/*
+	 * Directory entry creation below may acquire the AGF. Remove
+	 * the whiteout from the unlinked list first to preserve correct
+	 * AGI/AGF locking order.
+	 */
+	if (wip) {
+		ASSERT(VFS_I(wip)->i_nlink == 0);
+		error = xfs_iunlink_remove(tp, wip);
+		if (error)
+			goto out_trans_cancel;
+	}
+
+	/*
+	 * 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 +3347,6 @@ 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
@@ -3421,16 +3440,12 @@ struct xfs_iunlink {
  	 * 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.
+	 * the remaining reference to it. Move the xfs_iunlink_remove() call to
+	 * before acquiring the AGF lock to preserve correct AGI/AGF locking order.
  	 */
  	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);

  		/*
-- 
1.8.3.1

-- 
kaixuxia