Kernelspace C11 atomics for MIPS

[Adrian Chadd]

Hi,

It survived an overnight thrashing on my AR7161 (mips24k.)




Adrian

On 3 June 2013 22:07, Warner Losh <imp at bsdimp.com> wrote:
> Please find attached a simple patch that I'd like all MIPS users to try.
>
> Warner
>
>
>
>
> On Jun 3, 2013, at 10:15 PM, Patrick Kelsey wrote:
>
>> 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 :/
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>
>