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From: Matthew Macy <mmacy@freebsd.org>
Date: Fri, 8 Jun 2018 09:34:42 -0700
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Subject: Re: svn commit: r334827 - in head/sys: amd64/amd64 arm/arm dev/hwpmc
 i386/i386 kern mips/atheros mips/cavium powerpc/powerpc sys
To: Mark Johnston <markj@freebsd.org>
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> The fact that our NMI handler isn't re-entrant can lead to subtle
> problems. If while executing the NMI handler we hit a dtrace
> probe or DDB breakpoint, the iret executed upon return to the handler
> will re-enable NMIs. Then, if a second NMI arrives before the handler
> for the first has returned, the trapframe will be clobbered. Did you
> rule out an issue like this?

No, but it happened instantly on all CPUs an a non-debug kernel 100%
of the time after I changed pmc_process_interrupt earlier this week.
My voodoo fix now avoids it. What you're describing sounds episodic
and doesn't sound like it would be fixed / worked around by my change.

-M