linux-kernel - Re: BUG: using smp_processor_id() in preemptible arch_trigger_all_cpu_backtrace

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Date:	Mon, 1 Nov 2010 18:51:10 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Jan Kiszka <jan.kiszka@....de>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Eric Paris <eparis@...hat.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: BUG: using smp_processor_id() in preemptible
	arch_trigger_all_cpu_backtrace_handler

On Mon, Nov 01, 2010 at 11:45:36AM -0400, Don Zickus wrote:
...
> Heh.  Yeah when I migrated the code, I completely forgot the notifier
> chain could be called from a preemptible context (ie not NMI).
> 
> This patch should fix it and I think it is the correct fix.  Let me know
> how it works out.
> 
> Cheers,
> Don
> 
> diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
> index c7c9ae4..1bdd0b5 100644
> --- a/arch/x86/kernel/apic/hw_nmi.c
> +++ b/arch/x86/kernel/apic/hw_nmi.c
> @@ -63,7 +63,7 @@ arch_trigger_all_cpu_backtrace_handler(struct notifier_block *self,
>  {
>  	struct die_args *args = __args;
>  	struct pt_regs *regs;
> -	int cpu = smp_processor_id();
> +	int cpu;
>  
>  	switch (cmd) {
>  	case DIE_NMI:
> @@ -74,6 +74,7 @@ arch_trigger_all_cpu_backtrace_handler(struct notifier_block *self,
>  	}
>  
>  	regs = args->regs;
> +	cpu = smp_processor_id();
>  
>  	if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
>  		static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
> 

 yup, this will do the trick for a while. In general I believe we might have
kind of NMI exclusive chain so we wouldn't need the 'case:'s.

  Cyrill
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