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

From: Alexander Lobakin <aleksander.lobakin@intel.com>
To: Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
	"Helge Deller" <deller@gmx.de>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Paul Mackerras" <paulus@samba.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Borislav Petkov" <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Josh Poimboeuf" <jpoimboe@redhat.com>,
	"Jiri Kosina" <jikos@kernel.org>,
	"Miroslav Benes" <mbenes@suse.cz>,
	"Petr Mladek" <pmladek@suse.com>,
	"Joe Lawrence" <joe.lawrence@redhat.com>,
	"Colin Ian King" <colin.king@canonical.com>,
	"Masami Hiramatsu" <mhiramat@kernel.org>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Jisheng Zhang" <jszhang@kernel.org>,
	linux-csky@vger.kernel.org, linux-parisc@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
	live-patching@vger.kernel.org, 王贇 <yun.wang@linux.alibaba.com>,
	"Guo Ren" <guoren@kernel.org>, "Jakub Kicinski" <kuba@kernel.org>
Subject: Re: [PATCH] tracing: Have all levels of checks prevent recursion
Date: Fri, 21 Jul 2023 17:34:41 +0200	[thread overview]
Message-ID: <1b402c0c-1beb-d93f-ff6d-955350995ca3@intel.com> (raw)
In-Reply-To: <20211015142541.4badd8a9@gandalf.local.home>

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