Re: [PATCH v2 2/2] xen/evtchn: rework per event channel lock

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.10.2020 08:00, Jürgen Groß wrote:
> On 13.10.20 17:28, Jan Beulich wrote:
>> On 12.10.2020 11:27, Juergen Gross wrote:
>>> --- a/xen/include/xen/event.h
>>> +++ b/xen/include/xen/event.h
>>> @@ -105,6 +105,45 @@ void notify_via_xen_event_channel(struct domain *ld, 
>>> int lport);
>>>   #define bucket_from_port(d, p) \
>>>       ((group_from_port(d, p))[((p) % EVTCHNS_PER_GROUP) / 
>>> EVTCHNS_PER_BUCKET])
>>>   
>>> +#define EVENT_WRITE_LOCK_INC    MAX_VIRT_CPUS
>>
>> Isn't the ceiling on simultaneous readers the number of pCPU-s,
>> and the value here then needs to be NR_CPUS + 1 to accommodate
>> the maximum number of readers? Furthermore, with you dropping
>> the disabling of interrupts, one pCPU can acquire a read lock
>> now more than once, when interrupting a locked region.
> 
> Yes, I think you are right.
> 
> So at least 2 * (NR-CPUS + 1), or even 3 * (NR_CPUS + 1) for covering
> NMIs, too?

Hard to say: Even interrupts can in principle nest. I'd go further
and use e.g. INT_MAX / 4, albeit no matter what value we choose
there'll remain a theoretical risk. I'm therefore not fully
convinced of the concept, irrespective of it providing an elegant
solution to the problem at hand. I'd be curious what others think.

Jan