RE: [V4 PATCH 1/4] panic/x86: Fix re-entrance problem due to panic on NMI

[æåèå / KAWAIïHIDEHIRO]

Peter saids -tip tree doesn't have panic_on_unrecovered_nmi in the
previoius discussion, but it still exists.  So, I didn't change
anything about panic_on_unrecovered_nmi.

Thanks,

Hidehiro Kawai
Hitachi, Ltd. Research & Development Group

> From: Hidehiro Kawai [mailto:hidehiro.kawai.ez@xxxxxxxxxxx]
> 
> If panic on NMI happens just after panic() on the same CPU, panic()
> is recursively called.  As the result, it stalls after failing to
> acquire panic_lock.
> 
> To avoid this problem, don't call panic() in NMI context if
> we've already entered panic().
> 
> V4:
> - Improve comments in io_check_error() and panic()
> 
> V3:
> - Introduce nmi_panic() macro to reduce code duplication
> - In the case of panic on NMI, don't return from NMI handlers
>   if another cpu already panicked
> 
> V2:
> - Use atomic_cmpxchg() instead of current spin_trylock() to
>   exclude concurrent accesses to the panic routines
> - Don't introduce no-lock version of panic()
> 
> Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@xxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> ---
>  arch/x86/kernel/nmi.c  |   16 ++++++++++++----
>  include/linux/kernel.h |   13 +++++++++++++
>  kernel/panic.c         |   15 ++++++++++++---
>  kernel/watchdog.c      |    2 +-
>  4 files changed, 38 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
> index 697f90d..5131714 100644
> --- a/arch/x86/kernel/nmi.c
> +++ b/arch/x86/kernel/nmi.c
> @@ -231,7 +231,7 @@ void unregister_nmi_handler(unsigned int type, const char *name)
>  #endif
> 
>  	if (panic_on_unrecovered_nmi)
> -		panic("NMI: Not continuing");
> +		nmi_panic("NMI: Not continuing");
> 
>  	pr_emerg("Dazed and confused, but trying to continue\n");
> 
> @@ -255,8 +255,16 @@ void unregister_nmi_handler(unsigned int type, const char *name)
>  		 reason, smp_processor_id());
>  	show_regs(regs);
> 
> -	if (panic_on_io_nmi)
> -		panic("NMI IOCK error: Not continuing");
> +	if (panic_on_io_nmi) {
> +		nmi_panic("NMI IOCK error: Not continuing");
> +
> +		/*
> +		 * If we return from nmi_panic(), it means we have received
> +		 * NMI while processing panic().  So, simply return without
> +		 * a delay and re-enabling NMI.
> +		 */
> +		return;
> +	}
> 
>  	/* Re-enable the IOCK line, wait for a few seconds */
>  	reason = (reason & NMI_REASON_CLEAR_MASK) | NMI_REASON_CLEAR_IOCHK;
> @@ -297,7 +305,7 @@ void unregister_nmi_handler(unsigned int type, const char *name)
> 
>  	pr_emerg("Do you have a strange power saving mode enabled?\n");
>  	if (unknown_nmi_panic || panic_on_unrecovered_nmi)
> -		panic("NMI: Not continuing");
> +		nmi_panic("NMI: Not continuing");
> 
>  	pr_emerg("Dazed and confused, but trying to continue\n");
>  }
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 5582410..57c33da 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -443,6 +443,19 @@ extern __scanf(2, 0)
> 
>  extern bool crash_kexec_post_notifiers;
> 
> +extern atomic_t panic_cpu;
> +
> +/*
> + * A variant of panic() called from NMI context.
> + * If we've already panicked on this cpu, return from here.
> + */
> +#define nmi_panic(fmt, ...)						\
> +	do {								\
> +		int this_cpu = raw_smp_processor_id();			\
> +		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
> +			panic(fmt, ##__VA_ARGS__);			\
> +	} while (0)
> +
>  /*
>   * Only to be used by arch init code. If the user over-wrote the default
>   * CONFIG_PANIC_TIMEOUT, honor it.
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 04e91ff..a105e67 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -60,6 +60,8 @@ void __weak panic_smp_self_stop(void)
>  		cpu_relax();
>  }
> 
> +atomic_t panic_cpu = ATOMIC_INIT(-1);
> +
>  /**
>   *	panic - halt the system
>   *	@fmt: The text string to print
> @@ -70,17 +72,17 @@ void __weak panic_smp_self_stop(void)
>   */
>  void panic(const char *fmt, ...)
>  {
> -	static DEFINE_SPINLOCK(panic_lock);
>  	static char buf[1024];
>  	va_list args;
>  	long i, i_next = 0;
>  	int state = 0;
> +	int old_cpu, this_cpu;
> 
>  	/*
>  	 * Disable local interrupts. This will prevent panic_smp_self_stop
>  	 * from deadlocking the first cpu that invokes the panic, since
>  	 * there is nothing to prevent an interrupt handler (that runs
> -	 * after the panic_lock is acquired) from invoking panic again.
> +	 * after setting panic_cpu) from invoking panic again.
>  	 */
>  	local_irq_disable();
> 
> @@ -93,8 +95,15 @@ void panic(const char *fmt, ...)
>  	 * multiple parallel invocations of panic, all other CPUs either
>  	 * stop themself or will wait until they are stopped by the 1st CPU
>  	 * with smp_send_stop().
> +	 *
> +	 * `old_cpu == -1' means this is the 1st CPU which comes here, so
> +	 * go ahead.
> +	 * `old_cpu == this_cpu' means we came from nmi_panic() which sets
> +	 * panic_cpu to this cpu.  In this case, this is also the 1st CPU.
>  	 */
> -	if (!spin_trylock(&panic_lock))
> +	this_cpu = raw_smp_processor_id();
> +	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> +	if (old_cpu != -1 && old_cpu != this_cpu)
>  		panic_smp_self_stop();
> 
>  	console_verbose();
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 64ed1c3..00fbaa29 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -324,7 +324,7 @@ static void watchdog_overflow_callback(struct perf_event *event,
>  			return;
> 
>  		if (hardlockup_panic)
> -			panic("Watchdog detected hard LOCKUP on cpu %d",
> +			nmi_panic("Watchdog detected hard LOCKUP on cpu %d",
>  			      this_cpu);
>  		else
>  			WARN(1, "Watchdog detected hard LOCKUP on cpu %d",
> 

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