[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCHv3 2/6] evtchn: defer freeing struct evtchn's until evtchn_destroy_final()
On 19/06/15 14:04, Jan Beulich wrote: >>>> 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. I found the this is_dying check confusing and since it is now unnecessary and not very useful it is better to remove it to make the code easier for others to understand. David _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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