Date: Fri, 16 Nov 2012 14:41:52 +0000 From: Attilio Rao <attilio@freebsd.org> To: Andriy Gapon <avg@freebsd.org> Cc: freebsd-hackers@freebsd.org, Ryan Stone <rysto32@gmail.com> Subject: Re: stop_cpus_hard when multiple CPUs are panicking from an NMI Message-ID: <CAJ-FndDC1QCytXDJqVkism_5VoLNo_OzZxNEQ9NHx63HC=GTNg@mail.gmail.com> In-Reply-To: <50A63D1D.9090500@FreeBSD.org> References: <CAFMmRNwb_rxYXHGtXgtcyVUJnFDx5PSeMmA_crBbeV_rtzL9Cg@mail.gmail.com> <50A5F12C.1050902@FreeBSD.org> <CAJ-FndAB%2B7KRAE91L9634eXgzqgrizwtwCBC7AAg%2B0EX89TEBQ@mail.gmail.com> <50A63D1D.9090500@FreeBSD.org>
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On Fri, Nov 16, 2012 at 1:18 PM, Andriy Gapon <avg@freebsd.org> wrote: > on 16/11/2012 14:30 Attilio Rao said the following: >> On Fri, Nov 16, 2012 at 7:54 AM, Andriy Gapon <avg@freebsd.org> wrote: >>> 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. >> >> It is similar to what I propose but with some differences: >> - It is not clean from MI perspective > > OK. > >> - I don't think we need to treact it specially, I would just >> unconditionally stop all the CPUs entering in the "spinlock zone", >> making the patch simpler. > > I couldn't understand this part. I'm sorry, I think I misread your patch. I was basically suggesting to fix Ryan case by calling cpustop_handler() (or the new MI interface) into the panic() function, in the case the CPU don't win the race for panic_cpu. Basically doing: Index: sys/kern/kern_shutdown.c =================================================================== --- sys/kern/kern_shutdown.c (revision 243154) +++ sys/kern/kern_shutdown.c (working copy) @@ -568,15 +568,11 @@ panic(const char *fmt, ...) #ifdef SMP /* * 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. + * use panic_cpu as a simple lock. */ if (panic_cpu != PCPU_GET(cpuid)) - while (atomic_cmpset_int(&panic_cpu, NOCPU, - PCPU_GET(cpuid)) == 0) - while (panic_cpu != NOCPU) - ; /* nothing */ + if (atomic_cmpset_int(&panic_cpu, NOCPU, PCPU_GET(cpuid)) == 0) + cpustop_handler(); if (stop_scheduler_on_panic) { if (panicstr == NULL && !kdb_active) { Infact it seems to me that the comment is outdated and no longer represent truth. Attilio -- Peace can only be achieved by understanding - A. Einstein
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