netdev - Re: [PATCH net 7/9] rxrpc: Fix I/O thread stop

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Date:   Fri, 16 Dec 2022 06:46:01 +0000
From:   David Howells <dhowells@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     dhowells@...hat.com, netdev@...r.kernel.org,
        Marc Dionne <marc.dionne@...istor.com>,
        linux-afs@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 7/9] rxrpc: Fix I/O thread stop

Hillf Danton <hdanton@...a.com> wrote:

> > @@ -478,13 +479,14 @@ int rxrpc_io_thread(void *data)
> >  		}
> >  
> >  		set_current_state(TASK_INTERRUPTIBLE);
> > +		should_stop = kthread_should_stop();
> >  		if (!skb_queue_empty(&local->rx_queue) ||
> >  		    !list_empty(&local->call_attend_q)) {
> >  			__set_current_state(TASK_RUNNING);
> >  			continue;
> >  		}
> >  
> > -		if (kthread_should_stop())
> > +		if (should_stop)
> >  			break;
> >  		schedule();
> >  	}
> 
> At the second glance still fail to see the difference this change can
> make.

There is a window here:

  		if (!skb_queue_empty(&local->rx_queue) ...)
			continue;
	--->
		if (kthread_should_stop())
 			break;

in which an event can happen that should be attended to.  For instance the
AF_RXRPC socket being closed, aborting all its calls and stopping the kthread
by doing the final unuse on its rxrpc_local struct - in that order.  The
window can be expanded by an interrupt or softirq handler running.

So once we've observed that we've been asked to stop, we need to check if
there's more work to be done and, if so, do that work first.

So by doing:

		should_stop = kthread_should_stop();
  		if (!skb_queue_empty(&local->rx_queue) ...)
			continue;
		if (should_stop)
 			break;

we do all outstanding cleanup work before exiting the loop to stop the thread.

David