svn commit: r313268 - head/sys/kern [through -r313271 for atomic_fcmpset use and later: fails on PowerMac G5 "Quad Core"; -r313266 works]

[Mark Millard]

[Back to the powerpc64 context.]

On 2017-Feb-20, at 11:10 AM, Mateusz Guzik <mjguzik at gmail.com> wrote:

> On Sat, Feb 18, 2017 at 04:18:05AM -0800, Mark Millard wrote:
>> [Note: I experiment with clang based powerpc64 builds,
>> reporting problems that I find. Justin is familiar
>> with this, as is Nathan.]
>> 
>> I tried to update the PowerMac G5 (a so-called "Quad Core")
>> that I have access to from head -r312761 to -r313864 and
>> ended up with random panics and hang ups in fairly short
>> order after booting.
>> 
>> Some approximate bisecting for the kernel lead to:
>> (sometimes getting part way into a buildkernel attempt
>> for a different version before a failure happens)
>> 
>> -r313266: works (just before use of atomic_fcmpset)
>> vs.
>> -r313271: fails (last of the "use atomic_fcmpset" check-ins)
>> 
>> (I did not try -r313268 through -r313270 as the use was
>> gradually added.)
>> 
>> So I'm currently running a -r313864 world with a -r313266
>> kernel.
>> 
>> No kernel that I tried that was from before -r313266 had the
>> problems.
>> 
>> Any kernel that I tried that was from after -r313271 had the
>> problems.
>> 
>> Of course I did not try them all in other direction. :)
>> 
> 
> I found that spin mutexes were not properly handling this, fixed in
> r313996.
> 
> Locally I added a if (cpu_tick() % 2) return (0); snipped to amd64
> fcmpset to simulate failures. Everything works, while it would easily
> fail without the patch.
> 
> That said, I hope this concludes the 'missing check for not-reread value
> of failed fcmpset' saga.
> 
> -- 
> Mateusz Guzik <mjguzik gmail.com>

-r313999 is an improvement for powerpc64: it boots and I can
log in on the old PowerMac G5 so-called "Quad Core".

But, e.g., buildworld buildkernel eventually hangs and later
the powerpc64 panics for "spin lock held too long".

===
Mark Millard
markmi at dsl-only.net