Re: [Xen-devel] [V4 PATCH 2/2] mips/panic: Replace smp_send_stop() with kdump friendly version in panic path

[Corey Minyard <cminyard@xxxxxxxxxx>]

On 08/15/2016 12:06 PM, Corey Minyard wrote:
> On 08/15/2016 06:35 AM, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > Hi Corey,
> >
> > > From: Corey Minyard [mailto:cminyard@xxxxxxxxxx]
> > > Sent: Friday, August 12, 2016 10:56 PM
> > > I'll try to test this, but I have one comment inline...
> > Thank you very much!
> >
> > > On 08/11/2016 10:17 PM, Dave Young wrote:
> > > > On 08/10/16 at 05:09pm, Hidehiro Kawai wrote:
> > [snip]
> > > > > diff --git a/arch/mips/kernel/crash.c b/arch/mips/kernel/crash.c
> > > > > index 610f0f3..1723b17 100644
> > > > > --- a/arch/mips/kernel/crash.c
> > > > > +++ b/arch/mips/kernel/crash.c
> > > > > @@ -47,9 +47,14 @@ static void crash_shutdown_secondary(void 
> > > > > *passed_regs)
> > > > >    static void crash_kexec_prepare_cpus(void)
> > > > >    {
> > > > > +    static int cpus_stopped;
> > > > >        unsigned int msecs;
> > > > > +    unsigned int ncpus;
> > > > >
> > > > > -    unsigned int ncpus = num_online_cpus() - 1;/* Excluding the 
> > > > > panic cpu */
> > > > > +    if (cpus_stopped)
> > > > > +        return;
> > > Wouldn't you want an atomic operation and some special handling here to
> > > ensure that only one CPU does this?  So if a CPU comes in here and
> > > another CPU is already in the process stopping the CPUs it won't 
> > > result in a
> > > deadlock.
> > Because this function can be called only one panicking CPU,
> > there is no problem.
> >
> > There are two paths which crash_kexec_prepare_cpus is called.
> >
> > Path 1 (panic path):
> > panic()
> >    crash_smp_send_stop()
> >      crash_kexec_prepare_cpus()
> >
> > Path 2 (oops path):
> > crash_kexec()
> >    __crash_kexec()
> >      machine_crash_shutdown()
> >        default_machine_crash_shutdown() // for MIPS
> >          crash_kexec_prepare_cpus()
> >
> > Here, panic() and crash_kexec() run exclusively via
> > panic_cpu atomic variable.  So we can use cpus_stopped as
> > normal variable.
> Ok, if the code can only be entered once, what's the purpose of 
> cpus_stopped?
> I guess that's what confused me.  You are right, the panic_cpu atomic 
> should
> keep this on a single CPU.
Never mind, I see the path through panic() where that is required. My 
question
below still remains, though.

-corey

> Also, panic() will call panic_smp_self_stop() if it finds another CPU 
> has already
> called panic, which will just spin with interrupts off by default. I 
> didn't see a
> definition for it in MIPS, wouldn't it need to be overridden to avoid 
> a deadlock?
> -corey
>
> > Best regards,
> >
> > Hidehiro Kawai

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