[PATCH 37/75] SUNRPC: Prevent an rpc_task wakeup race

From: Kamal Mostafa
Date: Tue Jun 04 2013 - 13:06:27 EST -stable review patch. If anyone has any objections, please let me know.


From: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
net/sunrpc/sched.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c
index f8529fc..5356b12 100644
--- a/net/sunrpc/sched.c
+++ b/net/sunrpc/sched.c
@@ -324,11 +324,17 @@ EXPORT_SYMBOL_GPL(__rpc_wait_for_completion_task);
* 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);
- if (rpc_test_and_set_running(task))
+ if (!need_wakeup)
if (RPC_IS_ASYNC(task)) {
INIT_WORK(&task->u.tk_work, rpc_async_schedule);

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