[ 3.5.y.z extended stable ] Patch "SUNRPC: Add barriers to ensure read ordering in" has been added to staging queue

[Luis Henriques]

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

    SUNRPC: Add barriers to ensure read ordering in

to the linux-3.5.y-queue branch of the 3.5.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-3.5.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 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From a34ac4a9d0f6f9d8aff195e82187f052b95eb38a Mon Sep 17 00:00:00 2001
From: Trond Myklebust <Trond.Myklebust at netapp.com>
Date: Mon, 25 Mar 2013 11:23:40 -0400
Subject: [PATCH] SUNRPC: Add barriers to ensure read ordering in
 rpc_wake_up_task_queue_locked

commit 1166fde6a923c30f4351515b6a9a1efc513e7d00 upstream.

We need to be careful when testing task->tk_waitqueue in
rpc_wake_up_task_queue_locked, because it can be changed while we
are holding the queue->lock.
By adding appropriate memory barriers, we can ensure that it is safe to
test task->tk_waitqueue for equality if the RPC_TASK_QUEUED bit is set.

Signed-off-by: Trond Myklebust <Trond.Myklebust at netapp.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/sunrpc/sched.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index 85b9235..72d89e1 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -143,6 +143,8 @@ static void __rpc_add_wait_queue(struct rpc_wait_queue *queue,
 		list_add_tail(&task->u.tk_wait.list, &queue->tasks[0]);
 	task->tk_waitqueue = queue;
 	queue->qlen++;
+	/* barrier matches the read in rpc_wake_up_task_queue_locked() */
+	smp_wmb();
 	rpc_set_queued(task);

 	dprintk("RPC: %5u added to queue %p \"%s\"\n",
@@ -399,8 +401,11 @@ static void __rpc_do_wake_up_task(struct rpc_wait_queue *queue, struct rpc_task
  */
 static void rpc_wake_up_task_queue_locked(struct rpc_wait_queue *queue, struct rpc_task *task)
 {
-	if (RPC_IS_QUEUED(task) && task->tk_waitqueue == queue)
-		__rpc_do_wake_up_task(queue, task);
+	if (RPC_IS_QUEUED(task)) {
+		smp_rmb();
+		if (task->tk_waitqueue == queue)
+			__rpc_do_wake_up_task(queue, task);
+	}
 }

 /*
--
1.8.1.2