netdev - Re: [Xen-devel] [PATCH net v2 2/3] xen-netback: don't stop dealloc kthread too early

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Date:	Mon, 11 Aug 2014 15:34:48 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	<netdev@...r.kernel.org>, <xen-devel@...ts.xen.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Zoltan Kiss <zoltan.kiss@...rix.com>
Subject: Re: [Xen-devel] [PATCH net v2 2/3] xen-netback: don't stop dealloc
 kthread too early

On 11/08/14 15:31, Wei Liu wrote:
> On Mon, Aug 11, 2014 at 02:58:13PM +0100, David Vrabel wrote:
>> On 11/08/14 14:48, Wei Liu wrote:
>>> On Mon, Aug 11, 2014 at 01:10:12PM +0100, David Vrabel wrote:
>>>> On 08/08/14 17:37, Wei Liu wrote:
>>> [...]
>>>>>  	if (tx_evtchn == rx_evtchn) {
>>>>>  		/* feature-split-event-channels == 0 */
>>>>> @@ -687,6 +700,9 @@ void xenvif_disconnect(struct xenvif *vif)
>>>>>  			queue->task = NULL;
>>>>>  		}
>>>>>  
>>>>> +		wait_event(queue->inflight_wq,
>>>>> +			   atomic_read(&queue->inflight_packets) == 0);
>>>>
>>>> Just make the dealloc task not stop unless it has deallocated all
>>>> outstanding requests.  There's no need for another wait queue here.
>>>>
>>>
>>> Are you suggesting something like
>>>
>>>   while (atomic_read(&queue->inflight_packets) !=0)
>>>   	schedule_timeout(SOME_TIMEOUT);
>>
>> No. That would be awful!
>>
>> Add the test to the kthread itself:
>>
>> int xenvif_dealloc_kthread(void *data)
>> {
>> 	struct xenvif_queue *queue = data;
>>
>> 	while (atomic_read(&queue->inflight_packets) > 0
>>                || !kthread_should_stop()) {
>>             [...]
>>
>> etc.
>>
>> Although, the main loop is a bit confused, so I suggest adding:
>>
>> static bool xenvif_dealloc_thread_should_stop(struct xenvif_queue *q)
>> {
>>     /* Dealloc thread must remain running if there are any inflight
>>      * packets so it can properly dealloc them when they complete.
>>      */
>>     return atomic_read(&queue->inflight_packets) == 0
>>         && kthread_should_stop();
>> }
>>
>> And cleaning it up a bit (the while() could be a for(;;)).
>>
> 
> Unfortunately this approach is bogus. If xenbus thread is not blocked it
> can free up various resources while dealloc thread is running -- queue
> can be gone under dealloc thread's feet.

kthread_stop() waits until the thread exits (like pthread_join()).

/**
 * kthread_stop - stop a thread created by kthread_create().
 * @k: thread created by kthread_create().
 *
 * Sets kthread_should_stop() for @k to return true, wakes it, and
 * waits for it to exit.

David
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