Re: [PATCH] tracing: Have all levels of checks prevent recursion

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Steven Rostedt <rostedt@goodmis.org>
Date: Fri, 15 Oct 2021 14:25:41 -0400

Sorry for such a necroposting :z
Just wanted to know if this is a bug, so that I could send a fix, or
intended behaviour.

> On Fri, 15 Oct 2021 14:20:33 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
>>> I think having one copy of that in a header is better than having 3
>>> copies. But yes, something along them lines.  
>>
>> I was just about to ask you about this patch ;-)
> 
> Except it doesn't build :-p (need to move the inlined function down a bit)
> 
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 4d244e295e85..b32e3dabe28b 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -77,6 +77,27 @@
>  /* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */
>  #include <asm/preempt.h>
>  
> +/**
> + * interrupt_context_level - return interrupt context level
> + *
> + * Returns the current interrupt context level.
> + *  0 - normal context
> + *  1 - softirq context
> + *  2 - hardirq context
> + *  3 - NMI context
> + */
> +static __always_inline unsigned char interrupt_context_level(void)
> +{
> +	unsigned long pc = preempt_count();
> +	unsigned char level = 0;
> +
> +	level += !!(pc & (NMI_MASK));
> +	level += !!(pc & (NMI_MASK | HARDIRQ_MASK));
> +	level += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));

This doesn't take into account that we can switch the context manually
via local_bh_disable() / local_irq_save() etc. During the testing of the
separate issue[0], I've found that the function returns 1 in both just
softirq and softirq under local_irq_save().
Is this intended? Shouldn't that be

	level += !!(pc & (NMI_MASK));
	level += !!(pc * (NMI_MASK | HARDIRQ_MASK)) || irqs_disabled();
	level += !!(pc * (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)) ||
		 in_atomic();

?
Otherwise, the result it returns is not really "context level".

> +
> +	return level;
> +}
> +
[0]
https://lore.kernel.org/netdev/b3884ff9-d903-948d-797a-1830a39b1e71@intel.com

Thanks,
Olek