jbd2: fix fsync() tid wraparound bug

If an application program does not make any changes to the indirect
blocks or extent tree, i_datasync_tid will not get updated.  If there
are enough commits (i.e., 2**31) such that tid_geq()'s calculations
wrap, and there isn't a currently active transaction at the time of
the fdatasync() call, this can end up triggering a BUG_ON in
fs/jbd2/commit.c:

	J_ASSERT(journal->j_running_transaction != NULL);

It's pretty rare that this can happen, since it requires the use of
fdatasync() plus *very* frequent and excessive use of fsync().  But
with the right workload, it can.

We fix this by replacing the use of tid_geq() with an equality test,
since there's only one valid transaction id that we is valid for us to
wait until it is commited: namely, the currently running transaction
(if it exists).

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

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index e0ec3db..8c10004 100644 (file)
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -479,9 +479,12 @@ int __jbd2_log_space_left(journal_t *journal)
 int __jbd2_log_start_commit(journal_t *journal, tid_t target)
 {
        /*
-        * Are we already doing a recent enough commit?
+        * The only transaction we can possibly wait upon is the
+        * currently running transaction (if it exists).  Otherwise,
+        * the target tid must be an old one.
         */
-       if (!tid_geq(journal->j_commit_request, target)) {
+       if (journal->j_running_transaction &&
+           journal->j_running_transaction->t_tid == target) {
                /*
                 * We want a new commit: OK, mark the request and wakeup the
                 * commit thread.  We do _not_ do the commit ourselves.
@@ -493,7 +496,14 @@ int __jbd2_log_start_commit(journal_t *journal, tid_t target)
                          journal->j_commit_sequence);
                wake_up(&journal->j_wait_commit);
                return 1;
-       }
+       } else if (!tid_geq(journal->j_commit_request, target))
+               /* This should never happen, but if it does, preserve
+                  the evidence before kjournald goes into a loop and
+                  increments j_commit_sequence beyond all recognition. */
+               WARN(1, "jbd: bad log_start_commit: %u %u %u %u\n",
+                    journal->j_commit_request, journal->j_commit_sequence,
+                    target, journal->j_running_transaction ? 
+                    journal->j_running_transaction->t_tid : 0);
        return 0;
 }