Date:	Thu, 6 Oct 2011 16:50:42 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Robert Richter <robert.richter@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	seiji.aguchi@....com, vgoyal@...hat.com, mjg@...hat.com,
	tony.luck@...el.com, gong.chen@...el.com, satoru.moriya@....com,
	avi@...hat.com
Subject: Re: [RFC][PATCH] x86, reboot:  use NMI instead of REBOOT_VECTOR to
 stop cpus

On Wed, Sep 28, 2011 at 02:55:48PM -0400, Don Zickus wrote:
> A recent discussion started talking about the locking on the pstore fs
> and how it relates to the kmsg infrastructure.  We noticed it was possible
> for userspace to r/w to the pstore fs (grabbing the locks in the process)
> and block the panic path from r/w to the same fs.

Poke?  Anyone have an opinion on this?

Cheers,
Don

> 
> The reason was the cpu with the lock could be doing work while the crashing
> cpu is panic'ing.  Busting those spinlocks might cause those cpus to step
> on each other's data.  Fine, fair enough.
> 
> It was suggested it would be nice to serialize the panic path (ie stop
> the other cpus) and have only one cpu running.  This would allow us to
> bust the spinlocks and not worry about another cpu stepping on the data.
> 
> Of course, smp_send_stop() does this in the panic case.  kmsg_dump() would
> have to be moved to be called after it.  Easy enough.
> 
> The only problem is on x86 the smp_send_stop() function calls the
> REBOOT_VECTOR.  Any cpu with irqs disabled (which pstore and its backend
> ERST would do), block this IPI and thus do not stop.  This makes it
> difficult to reliably log data to the pstore fs.
> 
> The patch below switches from the REBOOT_VECTOR to NMI (and mimics what
> kdump does).  Switching to NMI allows us to deliver the IPI when irqs are
> disabled, increasing the reliability of this function.
> 
> However, Andi carefully noted that on some machines this approach does not
> work because of broken BIOSes or whatever.
> 
> I was hoping to get feedback on how much of a problem this really is.  Are
> there that many machines?  I assume most modern machines have a reliable NMI
> IPI mechanism (well on x86).  Is this just a problem on 32-bit machines?
> Early SMP machines?
> 
> One idea I had was to create a blacklist of machines and have those machines
> fallback to the original native_stop_other_cpus() that Andi wrote originally.
> The hope was that list was small.
> 
> Does anyone have any feedback whether this is a good idea or not?  Perhaps I am
> missing something?  Perhaps I should approach this problem differently?
> 
> [note] this patch sits on top of another NMI infrastructure change I have
> submitted, so the nmi registeration might not apply cleanly without that patch.
> However, for discussion purposes, I don't think that change is relevant, it is
> more the idea/philosophy of this patch that I am worried about.
> 
> Thanks,
> Don
> ---
>  arch/x86/kernel/smp.c |   56 +++++++++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 54 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 013e7eb..e98f0a1 100644
> --- a/arch/x86/kernel/smp.c
> +++ b/arch/x86/kernel/smp.c
> @@ -28,6 +28,7 @@
>  #include <asm/mmu_context.h>
>  #include <asm/proto.h>
>  #include <asm/apic.h>
> +#include <asm/nmi.h>
>  /*
>   *	Some notes on x86 processor bugs affecting SMP operation:
>   *
> @@ -147,6 +148,57 @@ void native_send_call_func_ipi(const struct cpumask *mask)
>  	free_cpumask_var(allbutself);
>  }
>  
> +static int stopping_cpu;
> +
> +static int smp_stop_nmi_callback(unsigned int val, struct pt_regs *regs)
> +{
> +	/* We are registerd on stopping cpu too, avoid spurious NMI */
> +	if (raw_smp_processor_id() == stopping_cpu)
> +		return NMI_HANDLED;
> +
> +	stop_this_cpu(NULL);
> +
> +	return NMI_HANDLED;
> +}
> +
> +static void native_nmi_stop_other_cpus(int wait)
> +{
> +	unsigned long flags;
> +	unsigned long timeout;
> +
> +	if (reboot_force)
> +		return;
> +
> +	/*
> +	 * Use an own vector here because smp_call_function
> +	 * does lots of things not suitable in a panic situation.
> +	 */
> +	if (num_online_cpus() > 1) {
> +		stopping_cpu = safe_smp_processor_id();
> +
> +		if (register_nmi_handler(NMI_LOCAL, smp_stop_nmi_callback,
> +					 NMI_FLAG_FIRST, "smp_stop"))
> +			return;		/* return what? */
> +
> +		/* sync above data before sending NMI */
> +		wmb();
> +
> +		apic->send_IPI_allbutself(NMI_VECTOR);
> +
> +		/*
> +		 * Don't wait longer than a second if the caller
> +		 * didn't ask us to wait.
> +		 */
> +		timeout = USEC_PER_SEC;
> +		while (num_online_cpus() > 1 && (wait || timeout--))
> +			udelay(1);
> +	}
> +
> +	local_irq_save(flags);
> +	disable_local_APIC();
> +	local_irq_restore(flags);
> +}
> +
>  /*
>   * this function calls the 'stop' function on all other CPUs in the system.
>   */
> @@ -159,7 +211,7 @@ asmlinkage void smp_reboot_interrupt(void)
>  	irq_exit();
>  }
>  
> -static void native_stop_other_cpus(int wait)
> +static void native_irq_stop_other_cpus(int wait)
>  {
>  	unsigned long flags;
>  	unsigned long timeout;
> @@ -229,7 +281,7 @@ struct smp_ops smp_ops = {
>  	.smp_prepare_cpus	= native_smp_prepare_cpus,
>  	.smp_cpus_done		= native_smp_cpus_done,
>  
> -	.stop_other_cpus	= native_stop_other_cpus,
> +	.stop_other_cpus	= native_nmi_stop_other_cpus,
>  	.smp_send_reschedule	= native_smp_send_reschedule,
>  
>  	.cpu_up			= native_cpu_up,
> -- 
> 1.7.6
> 
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