ext4: annotate data race in start_this_handle()

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
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 9396666..398d1d9 100644 (file)
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -349,7 +349,12 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
        }
 
 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.