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

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>