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Subject[58/85] jbd2: fix theoretical race in jbd2__journal_restart
3.2.49-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@mit.edu>

commit 39c04153fda8c32e85b51c96eb5511a326ad7609 upstream.

Once we decrement transaction->t_updates, if this is the last handle
holding the transaction from closing, and once we release the
t_handle_lock spinlock, it's possible for the transaction to commit
and be released. In practice with normal kernels, this probably won't
happen, since the commit happens in a separate kernel thread and it's
unlikely this could all happen within the space of a few CPU cycles.

On the other hand, with a real-time kernel, this could potentially
happen, so save the tid found in transaction->t_tid before we release
t_handle_lock. It would require an insane configuration, such as one
where the jbd2 thread was set to a very high real-time priority,
perhaps because a high priority real-time thread is trying to read or
write to a file system. But some people who use real-time kernels
have been known to do insane things, including controlling
laser-wielding industrial robots. :-)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/jbd2/transaction.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -470,10 +470,10 @@ int jbd2__journal_restart(handle_t *hand
&transaction->t_outstanding_credits);
if (atomic_dec_and_test(&transaction->t_updates))
wake_up(&journal->j_wait_updates);
+ tid = transaction->t_tid;
spin_unlock(&transaction->t_handle_lock);

jbd_debug(2, "restarting handle %p\n", handle);
- tid = transaction->t_tid;
need_to_start = !tid_geq(journal->j_commit_request, tid);
read_unlock(&journal->j_state_lock);
if (need_to_start)


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