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@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

1 files 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);