[ 3.8.y.z extended stable ] Patch "jbd2: fix theoretical race in jbd2__journal_restart" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    jbd2: fix theoretical race in jbd2__journal_restart

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 8a766a466098171f670beba8bc29366c9ff0cd54 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso at mit.edu>
Date: Mon, 1 Jul 2013 08:12:40 -0400
Subject: [PATCH] jbd2: fix theoretical race in jbd2__journal_restart

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 at mit.edu>
[ luis: backported to 3.8: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/jbd2/transaction.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index 73b9253..2f78ba3 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -500,10 +500,10 @@ int jbd2__journal_restart(handle_t *handle, int nblocks, gfp_t gfp_mask)
 		   &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)
--
1.8.1.2