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Subject[12/85] jbd: fix fsync() tid wraparound bug
2.6.33-longterm review patch.  If anyone has any objections, please let us know.

------------------
From: Ted Ts'o <tytso@mit.edu>

commit d9b01934d56a96d9f4ae2d6204d4ea78a36f5f36 upstream.

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/jbd/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 is valid for us to
start: namely, the currently running transaction (if it exists).

Reported-by: Martin_Zielinski@McAfee.com
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/jbd/journal.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
--- a/fs/jbd/journal.c
+++ b/fs/jbd/journal.c
@@ -435,9 +435,12 @@ int __log_space_left(journal_t *journal)
int __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 wakup the
* commit thread. We do _not_ do the commit ourselves.
@@ -449,7 +452,14 @@ int __log_start_commit(journal_t *journa
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_ONCE(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;
}




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