linux-ext4 - [PATCH 1/2] ext4: Annotate data race in start_this

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Message-Id: <20210406161804.20150-1-jack@suse.cz>
Date:   Tue,  6 Apr 2021 18:17:59 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ted Tso <tytso@....edu>
Cc:     <linux-ext4@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        syzbot+30774a6acf6a2cf6d535@...kaller.appspotmail.com
Subject: [PATCH 1/2] 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.

Reported-by: syzbot+30774a6acf6a2cf6d535@...kaller.appspotmail.com
Signed-off-by: Jan Kara <jack@...e.cz>
---
 fs/jbd2/transaction.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 9396666b7314..398d1d9209e2 100644
--- 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.
-- 
2.26.2