From: Jeff Mahoney Date: Mon, 23 Sep 2013 20:50:42 +0000 (-0400) Subject: reiserfs: fix race with flush_used_journal_lists and flush_journal_list X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=721a769c034204e8e5f6ca4ecbff249e0225333b;p=openwrt%2Fstaging%2Fblogic.git 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 Signed-off-by: Jan Kara --- diff --git a/fs/reiserfs/journal.c b/fs/reiserfs/journal.c index 7a37473b44dd..fd777032c2ba 100644 --- a/fs/reiserfs/journal.c +++ b/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; }