jbd2: Fix assertion failure in fs/jbd2/checkpoint.c

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 <jack@suse.cz>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index d5f7cff4cb28..d861ffd49821 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -442,6 +442,8 @@ struct transaction_s
         /*
          * Transaction's current state
          * [no locking - only kjournald2 alters this]
+         * [j_list_lock] guards transition of a transaction into T_FINISHED
+         * state and subsequent call of __jbd2_journal_drop_transaction()
          * FIXME: needs barriers
          * KLUDGE: [use j_state_lock]
          */