Re: [Xen-devel] [PATCHv3 2/6] evtchn: defer freeing struct evtchn's until evtchn_destroy_final()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.06.15 at 14:23, <david.vrabel@xxxxxxxxxx> wrote:
> On 19/06/15 11:55, Jan Beulich wrote:
>>>>> On 19.06.15 at 11:52, <david.vrabel@xxxxxxxxxx> wrote:
>>> On 19/06/15 10:29, Jan Beulich wrote:
>>>>>>> On 18.06.15 at 12:40, <david.vrabel@xxxxxxxxxx> wrote:
>>>>> On 18/06/15 11:36, Jan Beulich wrote:
>>>>>>>>> On 17.06.15 at 14:02, <david.vrabel@xxxxxxxxxx> wrote:
>>>>>>> --- a/xen/common/event_channel.c
>>>>>>> +++ b/xen/common/event_channel.c
>>>>>>> @@ -1175,22 +1175,6 @@ int alloc_unbound_xen_event_channel(
>>>>>>>  
>>>>>>>  void free_xen_event_channel(struct domain *d, int port)
>>>>>>>  {
>>>>>>> -    struct evtchn *chn;
>>>>>>> -
>>>>>>> -    spin_lock(&d->event_lock);
>>>>>>> -
>>>>>>> -    if ( unlikely(d->is_dying) )
>>>>>>> -    {
>>>>>>> -        spin_unlock(&d->event_lock);
>>>>>>> -        return;
>>>>>>> -    }
>>>>>>> -
>>>>>>> -    BUG_ON(!port_is_valid(d, port));
>>>>>
>>>>> I can keep this one.
>>>>>
>>>>>>> -    chn = evtchn_from_port(d, port);
>>>>>>> -    BUG_ON(!consumer_is_xen(chn));
>>>>>>
>>>>>> At least in debug builds I think these would better be retained.
>>>>>
>>>>> But this one has to go because it will always trip when
>>>>> free_xen_event_channel() is called after evtchn_destroy() (which will
>>>>> have cleared xen_consumer).
>>>>
>>>> Then why not
>>>>
>>>>     BUG_ON(!consumer_is_xen(chn) && !d->is_dying);
>>>>
>>>> or keep the d->is_dying check in place? I can see why accelerating
>>>> notify_via_xen_event_channel() is useful, but
>>>> free_xen_event_channel()?
>>>
>>> This BUG_ON() is a pretty weak check and I don't really see the point of
>>> it.  I'm not respinning v4 just for this.
>> 
>> I'm not sure what makes this more weak than the other BUG_ON()
>> you agreed to retain - both try to validate that the callers don't do
>> bad things. Admitted, both would better be ASSERT()s...
>> 
>> As to spinning v4 - I see no need, as I can easily adjust this while
>> committing, as long as you don't disagree to have your name under
>> the result.
> 
> I disagree.
> 
> For this assert to be safe it needs to take suitable locks such as:
> 
> #ifdef DEBUG
>     struct evtchn *chn;
> 
>     chn = evtchn_from_port(d, port);
>     spin_lock(&chn->lock);
>     BUG_ON(chn->state != ECS_FREE && !consumer_is_xen(chn));
>     spin_unlock(&chn->lock);
> #endif
> 
> or if you want the is_dying check, you need the event lock instead.
> 
> I wrote the first one, saw it was lots of noise for almost no gain and
> threw it away.

Which is why as an alternative I suggested not to touch
free_xen_event_channel() at all in this patch.

Jan


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