Re: [V5 PATCH 1/4] panic/x86: Fix re-entrance problem due to panic on NMI

[Borislav Petkov <bp@alien8.de>]

On Fri, Nov 20, 2015 at 06:36:44PM +0900, Hidehiro Kawai wrote:
> 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@hitachi.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> ---
>  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 @@ pci_serr_error(unsigned char reason, struct pt_regs *regs)
>  #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 @@ io_check_error(unsigned char reason, struct pt_regs *regs)
>  		 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");

Btw, that panic_on_io_nmi seems undocumented in
Documentation/sysctl/kernel.txt. Care to document it, please, as a
separate patch?

> +
> +		/*
> +		 * 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 @@ unknown_nmi_error(unsigned char reason, struct pt_regs *regs)
>  
>  	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 350dfb0..480a4fd 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
>  
>  extern bool crash_kexec_post_notifiers;
>  
> +extern atomic_t panic_cpu;

This needs a comment explaining what this variable is and what it
denotes.

> +
> +/*
> + * 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 4579dbb..24ee2ea 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -61,6 +61,8 @@ 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
> @@ -71,17 +73,17 @@ 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();
>  
> @@ -94,8 +96,15 @@ 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.

I'd prefer that -1 to be

#define INVALID_CPU_NUM		-1

or

#define UNDEFINED_CPU_NUM	-1

or so instead of a naked number.

-- 
Regards/Gruss,
    Boris.

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