Re: [PATCH v3 01/22] hardirq/nmi: Allow nested nmi_enter()

[Steven Rostedt]

On Wed, 19 Feb 2020 15:47:25 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> Since there are already a number of sites (ARM64, PowerPC) that
> effectively nest nmi_enter(), lets make the primitive support this
> before adding even more.
> 


>  void SMIException(struct pt_regs *regs)
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -71,7 +71,7 @@ extern void irq_exit(void);
>  		printk_nmi_enter();				\
>  		lockdep_off();					\
>  		ftrace_nmi_enter();				\
> -		BUG_ON(in_nmi());				\
> +		BUG_ON(in_nmi() == NMI_MASK);			\
>  		preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
>  		rcu_nmi_enter();				\
>  		trace_hardirq_enter();				\
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -26,13 +26,13 @@
>   *         PREEMPT_MASK:	0x000000ff
>   *         SOFTIRQ_MASK:	0x0000ff00
>   *         HARDIRQ_MASK:	0x000f0000
> - *             NMI_MASK:	0x00100000
> + *             NMI_MASK:	0x00f00000
>   * PREEMPT_NEED_RESCHED:	0x80000000
>   */
>  #define PREEMPT_BITS	8
>  #define SOFTIRQ_BITS	8
>  #define HARDIRQ_BITS	4
> -#define NMI_BITS	1
> +#define NMI_BITS	4
>  

Probably should document somewhere (in a comment above nmi_enter()?)
that we allow nmi_enter() to nest up to 15 times.

-- Steve