Patch "ext4: fix a race in the ext4 shutdown path" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

ext4: fix a race in the ext4 shutdown path

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ext4-fix-a-race-in-the-ext4-shutdown-path.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Harshad Shirwadkar <[email protected]>
Date: Wed, 10 Jan 2018 00:13:13 -0500
Subject: ext4: fix a race in the ext4 shutdown path

From: Harshad Shirwadkar <[email protected]>

commit abbc3f9395c76d554a9ed27d4b1ebfb5d9b0e4ca upstream.

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.

---
 fs/ext4/inode.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

@@ -3767,10 +3767,18 @@ static ssize_t ext4_direct_IO_write(stru
 		/* 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);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/ext4-fix-a-race-in-the-ext4-shutdown-path.patch