Kernelspace C11 atomics for MIPS

[Patrick Kelsey]

On Tue, Jun 4, 2013 at 12:08 AM, Patrick Kelsey <kelsey at ieee.org> wrote:
> On Mon, Jun 3, 2013 at 11:57 PM, Adrian Chadd <adrian at freebsd.org> wrote:
>> On 3 June 2013 20:55, Juli Mallett <jmallett at freebsd.org> wrote:
>>
>>> To drain the pipeline on certain deficient (and mostly older) CPUs by way of
>>> guesswork and a little vague magic.  Most CPUs we support, I would guess, do
>>> not need this, and it continues to exist solely for hysterical reasons.
>>
>> How can I turn it off for my compiles?
>>
>>> I've certainly gotten rid of them and some other cargo cult synchronization
>>> on Octeon for testing and had it survive under considerable load, and
>>> occasionally with some slight speedups (for some more commonly-used or
>>> slower things than Just a Bunch Of NOPs.)
>>
>> Right. Well, since it's happening on every inlined lock, it's a bit silly.
>>
>>> The trouble is that proving they aren't necessary requires being rigorous
>>> and careful in understanding documentation and errata, and FUD about their
>>> possible necessity is somewhat-intimidating.  It's not an easy kind of
>>> corruption/unreliability/etc., to prove the lack of empirically.
>>
>> I've checked the diassembly from gcc-4.mumble on linux; it doesn't
>> include NOPs like this as far as I can tell.
>>
>
> The sync + 8 nops is coming from the definition of mips_sync() in
> sys/mips/include/atomic.h.
>
> I agree with Juli that it appears to be a manual pipeline-flush
> holdover from earlier days - I'm guessing there's 8 nops because the
> R4000/4400 had both the sync instruction and an 8-stage pipeline.  I'm
> further guessing this was an attempt at providing stronger ordering
> semantics than the sync instruction itself for the following
> mb()/wmb()/rmb() definitions that use it, as the sync instruction
> definition doesn't restrict execution of the before/after loads/stores
> with respect to the sync instruction itself.

Forgot to emphasize that this particular bit of old-school
nop-counting is either pointless or a latent hazard - 8 does not cover
the deepest MIPS pipeline around, then there's superscalar issue to
consider - so I think it's either unnecessary or insufficient.  So
far, that's all criticism and no solution :/