Date: Fri, 16 Nov 2012 09:54:20 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: Ryan Stone <rysto32@gmail.com> Cc: freebsd-hackers@FreeBSD.org Subject: Re: stop_cpus_hard when multiple CPUs are panicking from an NMI Message-ID: <50A5F12C.1050902@FreeBSD.org> In-Reply-To: <CAFMmRNwb_rxYXHGtXgtcyVUJnFDx5PSeMmA_crBbeV_rtzL9Cg@mail.gmail.com> References: <CAFMmRNwb_rxYXHGtXgtcyVUJnFDx5PSeMmA_crBbeV_rtzL9Cg@mail.gmail.com>
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on 16/11/2012 00:58 Ryan Stone said the following: > At work we have some custom watchdog hardware that sends an NMI upon > expiry. We've modified the kernel to panic when it receives the watchdog > NMI. I've been trying the "stop_scheduler_on_panic" mode, and I've > discovered that when my watchdog expires, the system gets completely > wedged. After some digging, I've discovered is that I have multiple CPUs > getting the watchdog NMI and trying to panic concurrently. One of the CPUs > wins, and the rest spin forever in this code: > > /* > * We don't want multiple CPU's to panic at the same time, so we > * use panic_cpu as a simple spinlock. We have to keep checking > * panic_cpu if we are spinning in case the panic on the first > * CPU is canceled. > */ > if (panic_cpu != PCPU_GET(cpuid)) > while (atomic_cmpset_int(&panic_cpu, NOCPU, > PCPU_GET(cpuid)) == 0) > while (panic_cpu != NOCPU) > ; /* nothing */ > > The system wedges when stop_cpus_hard() is called, which sends NMIs to all > of the other CPUs and waits for them to acknowledge that they are stopped > before returning. However the CPU will not deliver an NMI to a CPU that is > already handling an NMI, so the other CPUs that got a watchdog NMI and are > spinning will never go into the NMI handler and acknowledge that they are > stopped. I thought about this issue and fixed (in my tree) in a different way: http://people.freebsd.org/~avg/cpu_stop-race.diff The need for spinlock_enter in the patch in not entirely clear. The main idea is that a CPU which calls cpu_stop and loses a race should voluntary enter cpustop_handler. I am also not sure about MI-cleanness of this patch. P.S. I also have the hard stop and the "soft" stop separate in my tree. Just in case there is a "simultaneous" occurrence of the soft stop happening for whatever reason and the hard stop for panic or kdb entry, I always want the hard stop to override the soft stop. > I've been able to work around this with the following hideous hack: > > --- kern_shutdown.c 2012-08-17 10:25:02.000000000 -0400 > +++ kern_shutdown.c 2012-11-15 17:04:10.000000000 -0500 > @@ -658,11 +658,15 @@ > * panic_cpu if we are spinning in case the panic on the first > * CPU is canceled. > */ > - if (panic_cpu != PCPU_GET(cpuid)) > + if (panic_cpu != PCPU_GET(cpuid)) { > while (atomic_cmpset_int(&panic_cpu, NOCPU, > - PCPU_GET(cpuid)) == 0) > + PCPU_GET(cpuid)) == 0) { > + atomic_set_int(&stopped_cpus, PCPU_GET(cpumask)); > while (panic_cpu != NOCPU) > ; /* nothing */ > + } > + atomic_clear_int(&stopped_cpus, PCPU_GET(cpumask)); > + } > > if (stop_scheduler_on_panic) { > if (panicstr == NULL && !kdb_active) > > > But I'm hoping that somebody has some ideas on a better way to fix this > kind of problem. -- Andriy Gapon
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