[Devel] [PATCH] nfs: protect callback execution against per-net callback thread shutdown

[Kirill Tkhai]

On 03.11.2017 19:47, Stanislav Kinsburskiy wrote:
> From: Stanislav Kinsburskiy <skinsbursky at parallels.com>
> 
> The problem is that per-net SUNRPC transports shutdown is done regardless
> current callback execution. This is a race leading to transport use-after-free
> in callback handler.

Could you please draw the race to show the interaction between functions?

> This patch fixes it in stright-forward way. I.e. it protects callback
> execution with the same mutex used for per-net data creation and destruction.
> Hopefully, it won't slow down NFS client significantly.
> 
> https://jira.sw.ru/browse/PSBM-75751
> 
> Signed-off-by: Stanislav Kinsburskiy <skinsbursky at parallels.com>
> ---
>  fs/nfs/callback.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
> index 0beb275..82e8ed1 100644
> --- a/fs/nfs/callback.c
> +++ b/fs/nfs/callback.c
> @@ -118,6 +118,7 @@ nfs41_callback_svc(void *vrqstp)
>  			continue;
>  
>  		prepare_to_wait(&serv->sv_cb_waitq, &wq, TASK_INTERRUPTIBLE);
> +		mutex_lock(&nfs_callback_mutex);
>  		spin_lock_bh(&serv->sv_cb_lock);
>  		if (!list_empty(&serv->sv_cb_list)) {
>  			req = list_first_entry(&serv->sv_cb_list,
> @@ -129,8 +130,10 @@ nfs41_callback_svc(void *vrqstp)
>  			error = bc_svc_process(serv, req, rqstp);
>  			dprintk("bc_svc_process() returned w/ error code= %d\n",
>  				error);
> +			mutex_unlock(&nfs_callback_mutex);
>  		} else {
>  			spin_unlock_bh(&serv->sv_cb_lock);
> +			mutex_unlock(&nfs_callback_mutex);
>  			schedule();
>  			finish_wait(&serv->sv_cb_waitq, &wq);
>  		}

Couldn't this change introduce a deadlock like below?
                                                          [thread]
nfs_callback_down()                                       nfs41_callback_svc()
   mutex_lock(&nfs_callback_mutex);                       
   kthread_stop(cb_info->task);                           
      wake_up_process();                                  
      wait_for_completion(&kthread->exited);              

                                                          mutex_lock(&nfs_callback_mutex);