reiserfs: fix race with flush_used_journal_lists and flush_journal_list

There are two locks involved in managing the journal lists. The general
reiserfs_write_lock and the journal->j_flush_mutex.

While flush_journal_list is sleeping to acquire the j_flush_mutex or to
submit a block for write, it will drop the write lock. This allows
another thread to acquire the write lock and ultimately call
flush_used_journal_lists to traverse the list of journal lists and
select one for flushing. It can select the journal_list that has just
had flush_journal_list called on it in the original thread and call it
again with the same journal_list.

The second thread then drops the write lock to acquire j_flush_mutex and
the first thread reacquires it and continues execution and eventually
clears and frees the journal list before dropping j_flush_mutex and
returning.

The second thread acquires j_flush_mutex and ends up operating on a
journal_list that has already been released. If the memory hasn't
been reused, we'll soon after hit a BUG_ON because the transaction id
has already been cleared. If it's been reused, we'll crash in other
fun ways.

Since flush_journal_list will synchronize on j_flush_mutex, we can fix
the race by taking a proper reference in flush_used_journal_lists
and checking to see if it's still valid after the mutex is taken. It's
safe to iterate the list of journal lists and pick a list with
just the write lock as long as a reference is taken on the journal list
before we drop the lock. We already have code to handle whether a
transaction has been flushed already so we can use that to handle the
race and get rid of the trans_id BUG_ON.
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/reiserfs/journal.c

@@ -1338,7 +1338,6 @@ static int flush_journal_list(struct super_block *s,
 		reiserfs_warning(s, "clm-2048", "called with wcount %d",
 				 atomic_read(&journal->j_wcount));
 	}
-	BUG_ON(jl->j_trans_id == 0);
 
 	/* if flushall == 0, the lock is already held */
 	if (flushall) {
@@ -1765,6 +1764,8 @@ static int flush_used_journal_lists(struct super_block *s,
 			break;
 		tjl = JOURNAL_LIST_ENTRY(tjl->j_list.next);
 	}
+	get_journal_list(jl);
+	get_journal_list(flush_jl);
 	/* try to find a group of blocks we can flush across all the
 	 ** transactions, but only bother if we've actually spanned
 	 ** across multiple lists
@@ -1773,6 +1774,8 @@ static int flush_used_journal_lists(struct super_block *s,
 		ret = kupdate_transactions(s, jl, &tjl, &trans_id, len, i);
 	}
 	flush_journal_list(s, flush_jl, 1);
+	put_journal_list(s, flush_jl);
+	put_journal_list(s, jl);
 	return 0;
 }