linux-ext4 - [PATCH] ext4: Fix a race in the ext4 shutdown path.

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Date:   Tue,  9 Jan 2018 14:52:57 -0800
From:   harshads <harshads@...gle.com>
To:     linux-ext4@...r.kernel.org
Cc:     tytso@....edu, harshads <harshads@...gle.com>
Subject: [PATCH] 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 <harshads@...gle.com>
---
 fs/ext4/inode.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 534a9130f625..4c2f8b57bdc7 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3767,10 +3767,18 @@ static ssize_t ext4_direct_IO_write(struct kiocb *iocb, struct iov_iter *iter)
 		/* 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);

-- 
2.16.0.rc1.238.g530d649a79-goog