linux-kernel - [59/78] RPC: killing RPC tasks races fixed

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Date:	Mon, 21 Mar 2011 16:02:15 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Stanislav Kinsbursky <skinsbursky@...nvz.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>
Subject: [59/78] RPC: killing RPC tasks races fixed

2.6.38-stable review patch.  If anyone has any objections, please let us know.

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

From: Stanislav Kinsbursky <skinsbursky@...allels.com>

commit 8e26de238fd794c8ea56a5c98bf67c40cfeb051d upstream.

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@...nvz.org>
Signed-off-by: Trond Myklebust <Trond.Myklebust@...app.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 net/sunrpc/clnt.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -436,7 +436,9 @@ void rpc_killall_tasks(struct rpc_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);


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