Re: [PATCH] x86: avoid calling arch_trigger_all_cpu_backtrace() atthe same time on SMP

[DDD]

Ingo Molnar wrote:
> * Don Zickus <dzickus@xxxxxxxxxx> wrote:
>
> > From: Dongdong Deng <dongdong.deng@xxxxxxxxxxxxx>
> >
> > The spin_lock_debug/rcu_cpu_stall detector uses
> > trigger_all_cpu_backtrace() to dump cpu backtrace.
> > Therefore it is possible that trigger_all_cpu_backtrace()
> > could be called at the same time on different CPUs, which
> > triggers and 'unknown reason NMI' warning. The following case
> > illustrates the problem:
> >
> >       CPU1                    CPU2                     ...   CPU N
> >                        trigger_all_cpu_backtrace()
> >                        set "backtrace_mask" to cpu mask
> >                                |
> > generate NMI interrupts  generate NMI interrupts       ...
> >     \                          |                               /
> >      \                         |                              /
> >    The "backtrace_mask" will be cleaned by the first NMI interrupt
> >   at nmi_watchdog_tick(), then the following NMI interrupts generated
> > by other cpus's arch_trigger_all_cpu_backtrace() will be took as
> > unknown reason NMI interrupts.
> >
> > This patch uses a lock to avoid the problem, and stop the
> > arch_trigger_all_cpu_backtrace() calling to avoid dumping double cpu
> > backtrace info when there is already a trigger_all_cpu_backtrace()
> > in progress.
> >
> > Signed-off-by: Dongdong Deng <dongdong.deng@xxxxxxxxxxxxx>
> > Reviewed-by: Bruce Ashfield <bruce.ashfield@xxxxxxxxxxxxx>
> > CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > CC: Ingo Molnar <mingo@xxxxxxxxxx>
> > CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > CC: x86@xxxxxxxxxx
> > CC: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Don Zickus <dzickus@xxxxxxxxxx>
> > ---
> >  arch/x86/kernel/apic/hw_nmi.c |   14 ++++++++++++++
> >  arch/x86/kernel/apic/nmi.c    |   14 ++++++++++++++
> >  2 files changed, 28 insertions(+), 0 deletions(-)
> >
> > diff --git a/arch/x86/kernel/apic/hw_nmi.c b/arch/x86/kernel/apic/hw_nmi.c
> > index cefd694..3aea0a5 100644
> > --- a/arch/x86/kernel/apic/hw_nmi.c
> > +++ b/arch/x86/kernel/apic/hw_nmi.c
> > @@ -29,6 +29,16 @@ u64 hw_nmi_get_sample_period(void)
> >  void arch_trigger_all_cpu_backtrace(void)
> >  {
> >  	int i;
> > +	static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
>
> Please dont put statics into the middle of local variables - put them into file 
> scope in a visible way.

Got it. will change it.

> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	if (!arch_spin_trylock(&lock))
> > +		/*
> > +		 * If there is already a trigger_all_cpu_backtrace()
> > +		 * in progress, don't output double cpu dump infos.
> > +		 */
> > +		goto out_restore_irq;
> >  
> >  	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
> >  
> > @@ -41,6 +51,10 @@ void arch_trigger_all_cpu_backtrace(void)
> >  			break;
> >  		mdelay(1);
> >  	}
> > +
> > +	arch_spin_unlock(&lock);
> > +out_restore_irq:
> > +	local_irq_restore(flags);
> >  }
> >  
> >  static int __kprobes
> > diff --git a/arch/x86/kernel/apic/nmi.c b/arch/x86/kernel/apic/nmi.c
> > index a43f71c..5fa8a13 100644
> > --- a/arch/x86/kernel/apic/nmi.c
> > +++ b/arch/x86/kernel/apic/nmi.c
> > @@ -552,6 +552,16 @@ int do_nmi_callback(struct pt_regs *regs, int cpu)
> >  void arch_trigger_all_cpu_backtrace(void)
> >  {
> >  	int i;
> > +	static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	if (!arch_spin_trylock(&lock))
> > +		/*
> > +		 * If there is already a trigger_all_cpu_backtrace()
> > +		 * in progress, don't output double cpu dump infos.
> > +		 */
> > +		goto out_restore_irq;
> >  
> >  	cpumask_copy(to_cpumask(backtrace_mask), cpu_online_mask);
> >  
> > @@ -564,4 +574,8 @@ void arch_trigger_all_cpu_backtrace(void)
> >  			break;
> >  		mdelay(1);
> >  	}
> > +
> > +	arch_spin_unlock(&lock);
> > +out_restore_irq:
> > +	local_irq_restore(flags);
>
> This spinlock is never actually used as a spinlock - it's a "in progress" flag. Why 
> not use a flag and test_and_set_bit()?

Yep, it's a "in progress" flag, I will change to use a flag to replace 
the spinlock.


> Also, the irq disabling really needed, will this code ever be called with irqs on?

This code could be called with irqs on.

If the arch_trigger_all_cpu_backtrace() was triggered by
"spin_lock()"'s spin_lock_debug detector, it is possible that
the irq is enabled, thus we have to save and disable it here.


Thanks,

Dongdong

> Thanks,
>
> 	Ingo

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