RPC: killing RPC tasks races fixed

RPC task RPC_TASK_QUEUED bit is set must be checked before trying to wake up
task rpc_killall_tasks() because task->tk_waitqueue can not be set (equal to
NULL).
Also, as Trond Myklebust mentioned, such approach (instead of checking
tk_waitqueue to NULL) allows us to "optimise away the call to
rpc_wake_up_queued_task() altogether for those
tasks that aren't queued".

Here is an example of dereferencing of tk_waitqueue equal to NULL:

CPU 0               	CPU 1				CPU 2
--------------------	---------------------	--------------------------
nfs4_run_open_task
rpc_run_task
rpc_execute
rpc_set_active
rpc_make_runnable
(waiting)
			rpc_async_schedule
			nfs4_open_prepare
			nfs_wait_on_sequence
						nfs_umount_begin
						rpc_killall_tasks
						rpc_wake_up_task
						rpc_wake_up_queued_task
						spin_lock(tk_waitqueue == NULL)
						BUG()
			rpc_sleep_on
			spin_lock(&q->lock)
			__rpc_sleep_on
			task->tk_waitqueue = q

Signed-off-by: Stanislav Kinsbursky <skinsbursky@openvz.org>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index edaf56e2ed29..e7a96e478f63 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -436,7 +436,9 @@ void rpc_killall_tasks(struct rpc_clnt *clnt)
                 if (!(rovr->tk_flags & RPC_TASK_KILLED)) {
                         rovr->tk_flags |= RPC_TASK_KILLED;
                         rpc_exit(rovr, -EIO);
-                        rpc_wake_up_queued_task(rovr->tk_waitqueue, rovr);
+                        if (RPC_IS_QUEUED(rovr))
+                                rpc_wake_up_queued_task(rovr->tk_waitqueue,
+                                                        rovr);
                 }
         }
         spin_unlock(&clnt->cl_lock);