linux-kernel - Re: [PATCH 1/3] x86, reboot: Use NMI instead of REBOOT

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Date:	Wed, 12 Oct 2011 10:35:42 +0800
From:	Chen Gong <gong.chen@...ux.intel.com>
To:	Don Zickus <dzickus@...hat.com>
CC:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	x86@...nel.org, 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: [PATCH 1/3] x86, reboot:  Use NMI instead of REBOOT_VECTOR to
 stop cpus

于 2011/10/11 23:24, Don Zickus 写道:
> 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.
>
> 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.
>
> To help accomodate this, the next couple of patches will run a selftest and
> provide a knob to disable.
>
> V2:
>    uses atomic ops to serialize the cpu that shuts everyone down
>
> Signed-off-by: Don Zickus<dzickus@...hat.com>
> ---
>
> [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.
> ---
>   arch/x86/kernel/smp.c |   58 +++++++++++++++++++++++++++++++++++++++++++++++-
>   1 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
> index 013e7eb..7bdbf6a 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 +148,59 @@ void native_send_call_func_ipi(const struct cpumask *mask)
>   	free_cpumask_var(allbutself);
>   }
>
> +static atomic_t stopping_cpu = ATOMIC_INIT(-1);
> +
> +static int smp_stop_nmi_callback(unsigned int val, struct pt_regs *regs)
> +{
> +	/* We are registered on stopping cpu too, avoid spurious NMI */
> +	if (raw_smp_processor_id() == atomic_read(&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) {
> +		/* did someone beat us here? */
> +		if (atomic_cmpxchg(&stopping_cpu, -1, safe_smp_processor_id() != -1))
> +			return;
> +
> +		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);

In this patch and next patch, how about using the same logic in commit 74d91e3c6

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