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
