Re: [PATCH v2 1/9] KVM: add kvm_request_pending

[Christoffer Dall <christoffer.dall@linaro.org>]

On Fri, Apr 07, 2017 at 11:33:33PM +0800, Paolo Bonzini wrote:
> 
> 
> On 06/04/2017 23:08, Andrew Jones wrote:
> > My own made-up lingo to state that each time the variable is accessed it
> > must be loaded anew, taken care of by the volatile use in READ_ONCE.  As
> > vcpu->requests can be written by other threads, then I prefer READ_ONCE
> > being used to read it, as it allows me to avoid spending energy convincing
> > myself that the compiler would have emitted a load at that point anyway.
> 
> Also, READ_ONCE without a barrier is really fishy unless it's
> 
>   while (READ_ONCE(x) != 2) {
>       ...
>   }
> 
> or similar, and WRITE_ONCE doesn't even have this exception.  So
> annotating variables accessed by multiple threads with
> READ_ONCE/WRITE_ONCE is generally a good idea.
> 

I'm sorry, I'm confused.  You're saying that it's fishy to use
READ_ONCE() without also having a barrier, but does that imply that if
you have a barrier, you should also have READ_ONCE() ?

In any case, as I hope I've made clear, I'm perfectly fine with having
READ_ONCE(), as long as we make a best effort attempt at describing why
we added it, so even I can understand that later - in the code directly
or in the commit message.

Thanks,
-Christoffer