linux-kernel - Re: [PATCHv2] arm64:kexec: have own crash_smp_send

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Date:   Thu, 17 Aug 2017 11:20:50 +0900
From:   Hoeun Ryu <hoeun.ryu@...il.com>
To:     James Morse <james.morse@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Robin Murphy <robin.murphy@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        David Daney <david.daney@...ium.com>,
        Rob Herring <robh@...nel.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] arm64:kexec: have own crash_smp_send_stop() for crash
 dump for nonpanic cores

Hello, James.

Thank you for the meticulous test and review.

On Fri, 2017-08-11 at 18:02 +0100, James Morse wrote:
> Hi Hoeun,
> 
> On 07/08/17 06:09, Hoeun Ryu wrote:
> > 
> >  Commit 0ee5941 : (x86/panic: replace smp_send_stop() with kdump friendly
> > version in panic path) introduced crash_smp_send_stop() which is a weak
> > function and can be overriden by architecture codes to fix the side effect
> (overridden)

It'll be fixed in the next version.

> 
> 
> > 
> > caused by commit f06e515 : (kernel/panic.c: add "crash_kexec_post_
> > notifiers" option).
> > 
> >  ARM64 architecture uses the weak version function and the problem is that
> > the weak function simply calls smp_send_stop() which makes other CPUs
> > offline and takes away the chance to save crash information for nonpanic
> > CPUs in machine_crash_shutdown() when crash_kexec_post_notifiers kernel
> > option is enabled.
> > 
> >  Calling smp_send_crash_stop() in machine_crash_shutdown() is useless
> > because all nonpanic CPUs are already offline by smp_send_stop() in this
> > case and smp_send_crash_stop() only works against online CPUs.
> 
> > 
> >  The result is that /proc/vmcore is not available with the error messages;
> > "Warning: Zero PT_NOTE entries found", "Kdump: vmcore not initialized".
> When I tried this I got one of these warnings for each secondary CPU, but the
> vmcore file was still available. When I ran 'crash' on the vmcore it reported:
> > 
> > CPUS: 6 [OFFLINE: 5]
> Did I miss as step to reproduce this? If not, can we change this paragraph to
> say something like:
> > 
> > The result is that secondary CPUs registers are not saved by crash_save_cpu()
> > and the vmcore file misreports these CPUs as being offline.

Actually the commit log comes from the patch to fix a similar issue in arm port.
I'll change the commit log with yours.

> 
> > 
> >  crash_smp_send_stop() is implemented to fix this problem by replacing the
> > exising smp_send_crash_stop() and adding a check for multiple calling to
> (existing)

It'll be fixed in the next version.

> 
> 
> > 
> > the function. The function (strong symbol version) saves crash information
> > for nonpanic CPUs and machine_crash_shutdown() tries to save crash
> > information for nonpanic CPUs only when crash_kexec_post_notifiers kernel
> > option is disabled.
> > 
> > * crash_kexec_post_notifiers : false
> > 
> >   panic()
> >     __crash_kexec()
> >       machine_crash_shutdown()
> >         crash_smp_send_stop()    <= save crash dump for nonpanic cores
> > 
> > * crash_kexec_post_notifiers : true
> > 
> >   panic()
> >     crash_smp_send_stop()        <= save crash dump for nonpanic cores
> >     __crash_kexec()
> >       machine_crash_shutdown()
> >         crash_smp_send_stop()    <= just return.
> 
> > 
> > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> > index dc66e6e..73d8f5e 100644
> > --- a/arch/arm64/kernel/smp.c
> > +++ b/arch/arm64/kernel/smp.c
> > @@ -977,11 +977,21 @@ void smp_send_stop(void)
> >  }
> >  
> >  #ifdef CONFIG_KEXEC_CORE
> > -void smp_send_crash_stop(void)
> > +void crash_smp_send_stop(void)
> >  {
> > +	static int cpus_stopped;
> >  	cpumask_t mask;
> >  	unsigned long timeout;
> >  
> > +	/*
> > +	 * This function can be called twice in panic path, but obviously
> > +	 * we execute this only once.
> > +	 */
> > +	if (cpus_stopped)
> > +		return;
> > +
> > +	cpus_stopped = 1;
> > +
> This cpus_stopped=1 can't happen on multiple CPUs at the same time as any second
> call is guaranteed to be on the same CPU, both are behind panic()s
> 'atomic_cmpxchg()'.

'cpu_stopped' variable is not for the race of multi CPUs.
This variable is simply to prevent from calling
'smp_cross_call(&mask, IPI_CPU_CRASH_STOP)' twice in the machine_crash_shutdown().
Please look at following call path.
 
* crash_kexec_post_notifiers : true
 panic()
     crash_smp_send_stop() {
         ...
         cpu_stopped = 1              <= make it '1'
         smp_cross_call()             <= save crash dump for nonpanic cores
     }
     __crash_kexec()
         machine_crash_shutdown()
             crash_smp_send_stop() { 
                 if (cpu_stopped)
                     return           <= just return.
             }
> 
> 
> Other than my '/proc/vmcore is not available' question above, this looks fine to me:
> Reviewed-by: James Morse <james.morse@....com>
> Tested-by: James Morse <james.morse@....com>
> 
> 
> Thanks!
> 
> James
> 
> 
>