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

[Corey Minyard]

On 08/15/2016 06:35 AM, æåèå / KAWAIïHIDEHIRO 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.

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