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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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.

David

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel