Re: [PATCH v2 6/8] evtchn: convert vIRQ lock to an r/w one

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.10.2020 11:57, Julien Grall wrote:
> On 20/10/2020 15:10, Jan Beulich wrote:
>> --- a/xen/common/event_channel.c
>> +++ b/xen/common/event_channel.c
>> @@ -449,6 +449,13 @@ int evtchn_bind_virq(evtchn_bind_virq_t
>>   
>>       spin_unlock_irqrestore(&chn->lock, flags);
>>   
>> +    /*
>> +     * If by any, the update of virq_to_evtchn[] would need guarding by
>> +     * virq_lock, but since this is the last action here, there's no strict
>> +     * need to acquire the lock. Hnece holding event_lock isn't helpful
> 
> s/Hnece/Hence/
> 
>> +     * anymore at this point, but utilize that its unlocking acts as the
>> +     * otherwise necessary smp_wmb() here.
>> +     */
>>       v->virq_to_evtchn[virq] = bind->port = port;
> 
> I think all access to v->virq_to_evtchn[virq] should use ACCESS_ONCE() 
> or {read, write}_atomic() to avoid any store/load tearing.

IOW you're suggesting this to be the subject of a separate patch?
I don't think such a conversion belongs here (nor even in this
series, seeing the much wider applicability of such a change
throughout the code base). Or are you seeing anything here which
would require such a conversion to be done as a prereq?

Jan