Re: [PATCH 11/12] evtchn: convert vIRQ lock to an r/w one

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.09.2020 19:18, Julien Grall wrote:
> On 29/09/2020 14:37, Jan Beulich wrote:
>> On 29.09.2020 15:03, Julien Grall wrote:
>>> I am thinking that it may be easier to hold the write lock when doing
>>> the update.
>>
>> ... perhaps this is indeed better. I have to admit that I never
>> fully understood the benefit of using spin_barrier() in this code
>> (as opposed to the use in evtchn_destroy()).
> 
> I am not entirely sure either. It looks like it is an attempt to make 
> v->virq_to_evtchn[X] visible without holding a lock.
> 
> Any holder of the lock after spin_barrier() has completed will read 0 
> and not try to use the lock.

I'm not sure I follow: A holder of the lock is obviously already
making use of the lock. Or are you talking of two different locks
here (recall that before XSA-343 there was just one lock involved
in sending)?

> But the update of v->virq_to_evtchn[X] should have used either 
> ACCESS_ONCE() or write_atomic().

Of course, like in so many other places in the code base.

Jan