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@xxxxxxxx>
---
fs/reiserfs/journal.c | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
--- a/fs/reiserfs/journal.c
+++ b/fs/reiserfs/journal.c
@@ -1363,7 +1363,6 @@ static int flush_journal_list(struct sup
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) {
@@ -1818,6 +1817,8 @@ static int flush_used_journal_lists(stru
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
@@ -1826,6 +1827,8 @@ static int flush_used_journal_lists(stru
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;
}
--
Jeff Mahoney
SUSE Labs
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