2007-11-28 14:34:11

by Jan Kara

[permalink] [raw]
Subject: [PATCH] Fix assertion failure in fs/jbd/checkpoint.c


the patch below should fix an assertion failure in JBD checkpointing
code. The patch survived some fsstress and similar runs on my test machine
so it shouldn't be obviously wrong ;).

Jan Kara <[email protected]>

Before we start committing a transaction, we call
__journal_clean_checkpoint_list() to cleanup transaction's written-back
buffers. If this call happens to remove all of them (and there were already
some buffers), __journal_remove_checkpoint() will decide to free the
transaction because it isn't (yet) a committing transaction and soon we fail
some assertion - the transaction really isn't ready to be freed :).
We change the check in __journal_remove_checkpoint() to free only a transaction
in T_FINISHED state. The locking there is subtle though (as everywhere in
JBD ;(). We use j_list_lock to protect the check and a subsequent call to
__journal_drop_transaction() and do the same in the end of
journal_commit_transaction() which is the only place where a transaction can
get to T_FINISHED state. Probably I'm too paranoid here and such locking is
not really necessary - checkpoint lists are processed only from
log_do_checkpoint() where a transaction must be already committed to be
processed or from __journal_clean_checkpoint_list() where kjournald itself
calls it and thus transaction cannot change state either. Better be safe if
something changes in future...

Signed-off-by: Jan Kara <[email protected]>

diff --git a/fs/jbd/checkpoint.c b/fs/jbd/checkpoint.c
index 47552d4..0f69c41 100644
--- a/fs/jbd/checkpoint.c
+++ b/fs/jbd/checkpoint.c
@@ -602,15 +602,15 @@ int __journal_remove_checkpoint(struct journal_head *jh)

* There is one special case to worry about: if we have just pulled the
- * buffer off a committing transaction's forget list, then even if the
- * checkpoint list is empty, the transaction obviously cannot be
- * dropped!
+ * buffer off a running or committing transaction's checkpoing list,
+ * then even if the checkpoint list is empty, the transaction obviously
+ * cannot be dropped!
- * The locking here around j_committing_transaction is a bit sleazy.
+ * The locking here around t_state is a bit sleazy.
* See the comment at the end of journal_commit_transaction().
- if (transaction == journal->j_committing_transaction) {
- JBUFFER_TRACE(jh, "belongs to committing transaction");
+ if (transaction->t_state != T_FINISHED) {
+ JBUFFER_TRACE(jh, "belongs to running/committing transaction");
goto out;

diff --git a/fs/jbd/commit.c b/fs/jbd/commit.c
index 8f1f2aa..610264b 100644
--- a/fs/jbd/commit.c
+++ b/fs/jbd/commit.c
@@ -858,10 +858,10 @@ restart_loop:
- * This is a bit sleazy. We borrow j_list_lock to protect
- * journal->j_committing_transaction in __journal_remove_checkpoint.
- * Really, __journal_remove_checkpoint should be using j_state_lock but
- * it's a bit hassle to hold that across __journal_remove_checkpoint
+ * This is a bit sleazy. We use j_list_lock to protect transition
+ * of a transaction into T_FINISHED state and calling
+ * __journal_drop_transaction(). Otherwise we could race with
+ * other checkpointing code processing the transaction...
diff --git a/include/linux/jbd.h b/include/linux/jbd.h
index 16e7ed8..d9ecd13 100644
--- a/include/linux/jbd.h
+++ b/include/linux/jbd.h
@@ -439,6 +439,8 @@ struct transaction_s
* Transaction's current state
* [no locking - only kjournald alters this]
+ * [j_list_lock] guards transition of a transaction into T_FINISHED
+ * state and subsequent call of __journal_drop_transaction()
* FIXME: needs barriers
* KLUDGE: [use j_state_lock]