Re: [RESEND PATCH v4] x86/hpet: Reduce HPET counter read contention

[Waiman Long]

On 08/12/2016 05:16 PM, Dave Hansen wrote:
> On 08/12/2016 01:18 PM, Andy Lutomirski wrote:
> > I don't think this is right.  If the HPET ever returns the same value
> > twice in a row (unlikely because it's generally too slow to read, but
> > it's plausible that someone will make a fast HPET some day), then this
> > could deadlock.
> True...
>
> I guess that means we've got to do some kind of sequence counter
> preferably in the same cacheline as the HPET value itself, or _something
> that we guarantee to change on each write to the cached value.

I have done something similar in the v5 patch that I just sent out.

> > Also, does this code need to be NMI-safe?  This implementation is
> > deadlocky if it's called from an NMI.
> Urg.  Can't we just do
>
> 	if (in_nmi())
> 		return read_real_hpet();
>
> ?

Yes, I am doing that in my v5 patch.

> > The original code was wait-free, right?  That was a nice property, too.
> You mean no spins?  I don't think this one really spins ever either.

In the contended case, the reader needs to wait until the new HPET value 
is available. I consider this a kind of waiting.

Cheers,
Longman