Subject: [v3,11/41] mips: reuse asm-generic/barrier.h

From: [email-masked] (Leonid Yegoshin)
Date: Tue, 12 Jan 2016 16:21:36 -0800

On 01/12/2016 01:40 PM, Peter Zijlstra wrote:
>
>> It is selectable only for MIPS R2 but not MIPS R6. The reason is - most of
>> MIPS R2 CPUs have short pipeline and that SYNC is just waste of CPU
>> resource, especially taking into account that "lightweight syncs" are
>> converted to a heavy "SYNC 0" in many of that CPUs. However the latest
>> MIPS/Imagination CPU have a pipeline long enough to hit a problem - absence
>> of SYNC at LL/SC inside atomics, barriers etc.
> What ?! Are you saying that because R2 has short pipelines its unlikely
> to hit the reordering issues and we can omit barriers?

It was my guess to explain - why barriers was not included originally. 
You can check with Ralf, he knows more about that time MIPS Linux code.

I bother with this more than 2 years and I just try to solve that issue 
- in recent CPUs the load after LL/SC synchronization instruction loop 
can get ahead of SC for sure, it was tested.

>
>>> And reading the MIPS64 v6.04 instruction set manual, I think 0x11/0x12
>>> are_NOT_  transitive and therefore cannot be used to implement the
>>> smp_mb__{before,after} stuff.
>>>
>>> That is, in MIPS speak, those SYNC types are Ordering Barriers, not
>>> Completion Barriers.
>> Please see above, point 2.
> That did not in fact enlighten things. Are they transitive/multi-copy
> atomic or not?

Peter Zijlstra recently wrote: "In particular we're very much all 
'confused' about the various notions of transitivity". I am actually 
confused too and need some examples here.

>
> (and here Will will go into great detail on the differences between the
> two and make our collective brains explode :-)
>
>>> That is, currently all architectures -- with exception of PPC -- have
>>> RCsc locks, but using these non-transitive things will get you RCpc
>>> locks.
>>>
>>> So yes, MIPS can go RCpc for its locks and share the burden of pain with
>>> PPC, but that needs to be a very concious decision.
>> I don't understand that - I tried hard but I can't find any word like
>> "RCsc", "RCpc" in Documents/ directory. Web search goes nowhere, of course.
> From: lkml.kernel.org/r/20150828153921.GF19282 at twins.programming.kicks-ass.net
>
> Yes, the difference between RCpc and RCsc is in the meaning of RELEASE +
> ACQUIRE. With RCsc that implies a full memory barrier, with RCpc it does
> not.

MIPS Arch starting from R2 requires that. If some CPU can't, it should 
execute a full "SYNC 0" instead, which is a full memory barrier.

>
> Currently PowerPC is the only arch that (can, and) does RCpc and gives a
> weaker RELEASE + ACQUIRE. Only the CPU who did the ACQUIRE is guaranteed
> to see the stores of the CPU which did the RELEASE in order.

Yes, it was a goal for SYNC_ACQUIRE and SYNC_RELEASE.

Caveats:

     - "Full memory barrier" on MIPS means - full barrier for any device 
in coherent domain. In MIPS Tech/Imagination Tech MIPS-based CPU it is 
"for any device connected to CM or IOCU + directly connected memory".

     - It is not applied to instruction fetch. However, I-Cache flushes 
and SYNCI are consistent with that. There is also hazard barrier 
instructions to clear CPU pipeline to some extent - to help with this 
limitation.

I don't think that these caveats prevent a correct Acquire/Release semantic.

- Leonid.