Re: [PATCH V2] MIPS: implement smp_cond_load_acquire() for Loongson-3

[Jiaxun Yang]

On 2018-7-10 Tue at 20:17:27ïPeter Zijlstra Wroteï

Hi Peter
Since Huacai unable to send email via client, I'm going to reply for him 
 
> Sure.. we all got that far. And no, this isn't the _real_ problem. This
> is a manifestation of the problem.
> 
> The problem is that your SFB is broken (per the Linux requirements). We
> require that stores will become visible. That is, they must not
> indefinitely (for whatever reason) stay in the store buffer.
> 
> > I don't think this is a hardware bug, in design, SFB will flushed to
> > L1 cache in three cases:
> > 
> > 1, data in SFB is full (be a complete cache line);
> > 2, there is a subsequent read access in the same cache line;
> > 3, a 'sync' instruction is executed.
> 
> And I think this _is_ a hardware bug. You just designed the bug instead
> of it being by accident.
Yes, we understood that this hardware feature is not supported by LKML,
so it should be a hardware bug for LKML.
> 
> It doesn't happen an _any_ other architecture except that dodgy
> ARM11MPCore part. Linux hard relies on stores to become available
> _eventually_.
> 
> Still, even with the rules above, the best work-around is still the very
> same cpu_relax() hack.

As you say, SFB makes Loongson not fully SMP-coherent.
However, modify cpu_relax can solve the current problem,
but not so straight forward. On the other hand, providing a Loongson-specific 
WRITE_ONCE looks more reasonable, because it the eliminate the "non-cohrency".
So we can solve the bug from the root.

Thanks.
--
Jiaxun Yang