From: Jan Kara <jack@suse.cz>

commit 3b1833e92baba135923af4a07e73fe6e54be5a2f upstream.

Access to journal->j_running_transaction is not protected by appropriate
lock and thus is racy. We are well aware of that and the code handles
the race properly. Just add a comment and data_race() annotation.

Cc: stable@kernel.org
Reported-by: syzbot+30774a6acf6a2cf6d535@syzkaller.appspotmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210406161804.20150-1-jack@suse.cz (local)
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/jbd2/transaction.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c

@@ -349,7 +349,12 @@ static int start_this_handle(journal_t *
 	}
 
 alloc_transaction:
-	if (!journal->j_running_transaction) {
+	/*
+	 * This check is racy but it is just an optimization of allocating new
+	 * transaction early if there are high chances we'll need it. If we
+	 * guess wrong, we'll retry or free unused transaction.
+	 */
+	if (!data_race(journal->j_running_transaction)) {
 		/*
 		 * If __GFP_FS is not present, then we may be being called from
 		 * inside the fs writeback layer, so we MUST NOT fail.