linux-kernel - [PATCH 3.4 78/99] SUNRPC: Prevent an rpc

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Message-Id: <20140308010614.094996541@linuxfoundation.org>
Date:	Fri,  7 Mar 2014 17:08:14 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Weng Meiling <wengmeiling.weng@...wei.com>
Subject: [PATCH 3.4 78/99] SUNRPC: Prevent an rpc_task wakeup race

3.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Trond Myklebust <Trond.Myklebust@...app.com>

commit a3c3cac5d31879cd9ae2de7874dc6544ca704aec upstream.

The lockless RPC_IS_QUEUED() test in __rpc_execute means that we need to
be careful about ordering the calls to rpc_test_and_set_running(task) and
rpc_clear_queued(task). If we get the order wrong, then we may end up
testing the RPC_TASK_RUNNING flag after __rpc_execute() has looped
and changed the state of the rpc_task.

Signed-off-by: Trond Myklebust <Trond.Myklebust@...app.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
Cc: Weng Meiling <wengmeiling.weng@...wei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 net/sunrpc/sched.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -305,11 +305,17 @@ EXPORT_SYMBOL_GPL(__rpc_wait_for_complet
  * Note: If the task is ASYNC, and is being made runnable after sitting on an
  * rpc_wait_queue, this must be called with the queue spinlock held to protect
  * the wait queue operation.
+ * Note the ordering of rpc_test_and_set_running() and rpc_clear_queued(),
+ * which is needed to ensure that __rpc_execute() doesn't loop (due to the
+ * lockless RPC_IS_QUEUED() test) before we've had a chance to test
+ * the RPC_TASK_RUNNING flag.
  */
 static void rpc_make_runnable(struct rpc_task *task)
 {
+	bool need_wakeup = !rpc_test_and_set_running(task);
+
 	rpc_clear_queued(task);
-	if (rpc_test_and_set_running(task))
+	if (!need_wakeup)
 		return;
 	if (RPC_IS_ASYNC(task)) {
 		INIT_WORK(&task->u.tk_work, rpc_async_schedule);


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