Date: Mon, 20 Feb 2017 20:10:45 +0100 From: Mateusz Guzik <mjguzik@gmail.com> To: Mark Millard <markmi@dsl-only.net> Cc: mjg@freebsd.org, Justin Hibbits <chmeeedalf@gmail.com>, svn-src-head@freebsd.org, FreeBSD PowerPC ML <freebsd-ppc@freebsd.org>, FreeBSD Current <freebsd-current@freebsd.org>, Nathan Whitehorn <nwhitehorn@freebsd.org> Subject: Re: svn commit: r313268 - head/sys/kern [through -r313271 for atomic_fcmpset use and later: fails on PowerMac G5 "Quad Core"; -r313266 works] Message-ID: <20170220191044.GA8526@dft-labs.eu> In-Reply-To: <2FD12B8F-2255-470A-98D4-2DCE9C7495F5@dsl-only.net> References: <2FD12B8F-2255-470A-98D4-2DCE9C7495F5@dsl-only.net>
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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>
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