Date: Mon, 3 Jun 2013 23:07:41 -0600 From: Warner Losh <imp@bsdimp.com> To: Patrick Kelsey <kelsey@ieee.org> Cc: Ed Schouten <ed@80386.nl>, "freebsd-mips@FreeBSD.org" <freebsd-mips@freebsd.org>, FreeBSD-arch <freebsd-arch@freebsd.org> Subject: Re: Kernelspace C11 atomics for MIPS Message-ID: <232DBBD8-3F32-4D42-85AB-AC5647EEA768@bsdimp.com> In-Reply-To: <CAD44qMV2indeyD-OsJsyR_yp-HowJOrKG6Znx3zHkSKzt7xQzA@mail.gmail.com> References: <CAJOYFBD502MYbkVR2hnVDTYWOvOUr15=OPyjotNvv%2BZ09vQ1OQ@mail.gmail.com> <D02AF210-5129-40AB-9481-3F0A44575E98@bsdimp.com> <CAJ-Vmo=vNbT9majPCZ8ugzPsNzh46DTD4mMDX-cuxx9Og91ptw@mail.gmail.com> <CACVs6=_X5vOfR%2BQOgvz6P-j3jUoNoK9hCFvz80fGRL3-PgBf5g@mail.gmail.com> <CAJ-Vmo=5%2BmWk4EWBuTdpF6vKx-%2BK=g1euJvZuRDF%2BvFkJNZZ4A@mail.gmail.com> <CAD44qMUUQ6_FDiTknK1rcW=rs7Mz96X35QgnRxH%2Br5=ZqGRbAw@mail.gmail.com> <CAD44qMV2indeyD-OsJsyR_yp-HowJOrKG6Znx3zHkSKzt7xQzA@mail.gmail.com>
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[-- Attachment #1 --]
Please find attached a simple patch that I'd like all MIPS users to try.
Warner
[-- Attachment #2 --]
Index: atomic.h
===================================================================
--- atomic.h (revision 250753)
+++ atomic.h (working copy)
@@ -44,20 +44,16 @@
* do not have atomic operations defined for them, but generally shouldn't
* need atomic operations.
*/
+#ifndef __MIPS_PLATFORM_SYNC_NOPS
+#define __MIPS_PLATFORM_SYNC_NOPS ""
+#endif
static __inline void
mips_sync(void)
{
- __asm __volatile (".set noreorder\n\t"
- "sync\n\t"
- "nop\n\t"
- "nop\n\t"
- "nop\n\t"
- "nop\n\t"
- "nop\n\t"
- "nop\n\t"
- "nop\n\t"
- "nop\n\t"
+ __asm __volatile (".set noreorder\n"
+ "\tsync\n"
+ __MIPS_PLATFORM_SYNC_NOPS
".set reorder\n"
: : : "memory");
}
[-- Attachment #3 --]
On Jun 3, 2013, at 10:15 PM, Patrick Kelsey wrote:
> On Tue, Jun 4, 2013 at 12:08 AM, Patrick Kelsey <kelsey@ieee.org> wrote:
>> On Mon, Jun 3, 2013 at 11:57 PM, Adrian Chadd <adrian@freebsd.org> wrote:
>>> On 3 June 2013 20:55, Juli Mallett <jmallett@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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