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

[河合英宏 / KAWAI，HIDEHIRO <hidehiro.kawai.ez@xxxxxxxxxxx>]

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.

Best regards,

Hidehiro Kawai

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