ext4/jbd2: don't wait (forever) for stale tid caused by wraparound

In the case where an inode has a very stale transaction id (tid) in
i_datasync_tid or i_sync_tid, it's possible that after a very large
(2**31) number of transactions, that the tid number space might wrap,
causing tid_geq()'s calculations to fail.

Commit deeeaf13 "jbd2: fix fsync() tid wraparound bug", later modified
by commit e7b04ac0 "jbd2: don't wake kjournald unnecessarily",
attempted to fix this problem, but it only avoided kjournald spinning
forever by fixing the logic in jbd2_log_start_commit().

Unfortunately, in the codepaths in fs/ext4/fsync.c and fs/ext4/inode.c
that might call jbd2_log_start_commit() with a stale tid, those
functions will subsequently call jbd2_log_wait_commit() with the same
stale tid, and then wait for a very long time.  To fix this, we
replace the calls to jbd2_log_start_commit() and
jbd2_log_wait_commit() with a call to a new function,
jbd2_complete_transaction(), which will correctly handle stale tid's.

As a bonus, jbd2_complete_transaction() will avoid locking
j_state_lock for writing unless a commit needs to be started.  This
should have a small (but probably not measurable) improvement for
ext4's scalability.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Reported-by: George Barnett <gbarnett@atlassian.com>
Cc: stable@vger.kernel.org

1 files changed, 31 insertions, 0 deletions

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index ed10991ab006..886ec2faa9b4 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -710,6 +710,37 @@ int jbd2_log_wait_commit(journal_t *journal, tid_t tid)
 }
 
 /*
+ * When this function returns the transaction corresponding to tid
+ * will be completed.  If the transaction has currently running, start
+ * committing that transaction before waiting for it to complete.  If
+ * the transaction id is stale, it is by definition already completed,
+ * so just return SUCCESS.
+ */
+int jbd2_complete_transaction(journal_t *journal, tid_t tid)
+{
+        int     need_to_wait = 1;
+
+        read_lock(&journal->j_state_lock);
+        if (journal->j_running_transaction &&
+            journal->j_running_transaction->t_tid == tid) {
+                if (journal->j_commit_request != tid) {
+                        /* transaction not yet started, so request it */
+                        read_unlock(&journal->j_state_lock);
+                        jbd2_log_start_commit(journal, tid);
+                        goto wait_commit;
+                }
+        } else if (!(journal->j_committing_transaction &&
+                     journal->j_committing_transaction->t_tid == tid))
+                need_to_wait = 0;
+        read_unlock(&journal->j_state_lock);
+        if (!need_to_wait)
+                return 0;
+wait_commit:
+        return jbd2_log_wait_commit(journal, tid);
+}
+EXPORT_SYMBOL(jbd2_complete_transaction);
+
+/*
  * Log buffer allocation routines:
  */