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

[Mateusz Guzik]

On Tue, Feb 21, 2017 at 01:37:25AM -0800, Mark Millard wrote:
> [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".
> 

Allright, play time is over.

Can you please:
1. verify r313254 is stable for you
2. apply https://people.freebsd.org/~mjg/patches/complete-locks.diff and
https://people.freebsd.org/~mjg/.junk/ppc.diff on top of it and retry
the test?

This is a workaround which effectively disables the powerpc-specific
primitive and makes it use a cmpset wrapper instead. I don't have the
hardware to test right now and my attempts to boot in qemu also failed.

That said, does not look like there are general fcmpset bugs left and
the remaining issue seems powerpc-specific.

If this works, I'll commit the workaround for the time being as in few
weeks I'd like to start merging the work back to stable/11.

-- 
Mateusz Guzik <mjguzik gmail.com>