ext4: fix a race in the ext4 shutdown path

This patch fixes a race between the shutdown path and bio completion
handling. In the ext4 direct io path with async io, after submitting a
bio to the block layer, if journal starting fails,
ext4_direct_IO_write() would bail out pretending that the IO
failed. The caller would have had no way of knowing whether or not the
IO was successfully submitted. So instead, we return -EIOCBQUEUED in
this case. Now, the caller knows that the IO was submitted. The bio
completion handler takes care of the error.

Tested: Ran the shutdown xfstest test 461 in loop for over 2 hours across
4 machines resulting in over 400 runs. Verified that the race didn't
occur. Usually the race was seen in about 20-30 iterations.

Signed-off-by: Harshad Shirwadkar <[e-mail]>
Signed-off-by: Theodore Ts'o <[e-mail]>
Cc: [e-mail]

fs/ext4/inode.c

···
 		/* Credits for sb + inode write */
 		handle = ext4_journal_start(inode, EXT4_HT_INODE, 2);
 		if (IS_ERR(handle)) {
-			/* This is really bad luck. We've written the data
-			 * but cannot extend i_size. Bail out and pretend
-			 * the write failed... */
-			ret = PTR_ERR(handle);
+			/*
+			 * We wrote the data but cannot extend
+			 * i_size. Bail out. In async io case, we do
+			 * not return error here because we have
+			 * already submmitted the corresponding
+			 * bio. Returning error here makes the caller
+			 * think that this IO is done and failed
+			 * resulting in race with bio's completion
+			 * handler.
+			 */
+			if (!ret)
+				ret = PTR_ERR(handle);
 			if (inode->i_nlink)
 				ext4_orphan_del(NULL, inode);