Re: [PATCH] RPC: killing RPC tasks races fixed

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Thu, 2011-03-17 at 15:16 +0300, Stanislav Kinsbursky wrote:
> task->tk_waitqueue must be checked for NULL before trying to wake up task in
> rpc_killall_tasks() because it can be NULL.
> 
> Here is an example:
> 
> 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>
> 
> ---
>  net/sunrpc/clnt.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
> index 57d344c..24039fe 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 (rovr->tk_waitqueue)
> +				rpc_wake_up_queued_task(rovr->tk_waitqueue, 
> +							rovr);

Testing for RPC_IS_QUEUED(rovr) would be better, since that would
optimise away the call to rpc_wake_up_queued_task() altogether for those
tasks that aren't queued.

>  		}
>  	}
>  	spin_unlock(&clnt->cl_lock);
> 

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@netapp.com
www.netapp.com