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

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Message-ID: <035cee53-255b-11a3-d7ac-ca46c05b907b@intel.com>
Date:   Fri, 21 Jul 2023 18:06:07 +0200
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Benjamin Herrenschmidt" <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "Jiri Kosina" <jikos@...nel.org>, Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Colin Ian King <colin.king@...onical.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Nicholas Piggin" <npiggin@...il.com>,
        Jisheng Zhang <jszhang@...nel.org>,
        <linux-csky@...r.kernel.org>, <linux-parisc@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <linux-riscv@...ts.infradead.org>,
        <live-patching@...r.kernel.org>,
        王贇 <yun.wang@...ux.alibaba.com>,
        Guo Ren <guoren@...nel.org>, Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH] tracing: Have all levels of checks prevent recursion

From: Steven Rostedt <rostedt@...dmis.org>
Date: Fri, 21 Jul 2023 12:00:40 -0400

> On Fri, 21 Jul 2023 17:34:41 +0200
> Alexander Lobakin <aleksander.lobakin@...el.com> wrote:

[...]

>>> +	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
> 
> You cannot manually switch interrupt context.
> 
>> 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
> 
> That is intended behavior.
> 
> local_bh_disable() and local_irq_save() is not a context switch. It is just
> preventing that context from happening. The interrupt_context_level() is to
> tell us what context we are running in, not what context is disabled.
> 
>>
>> 	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".
> 
> local_bh_disable() use to (and perhaps still does in some configurations)
> confuse things. But read the comment in kernel/softirq.c
> 
> /*
>  * SOFTIRQ_OFFSET usage:
>  *
>  * On !RT kernels 'count' is the preempt counter, on RT kernels this applies
>  * to a per CPU counter and to task::softirqs_disabled_cnt.
>  *
>  * - count is changed by SOFTIRQ_OFFSET on entering or leaving softirq
>  *   processing.
>  *
>  * - count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
>  *   on local_bh_disable or local_bh_enable.
>  *
>  * This lets us distinguish between whether we are currently processing
>  * softirq and whether we just have bh disabled.
>  */
> 
> Just because you disable interrupts does not mean you are in interrupt
> context.

Ah okay, thanks! IOW, if we want to check in some code that we're
certainly have interrupts enabled and are not in the interrupt context,
we must always do

if (!(in_hardirq() || irqs_disabled()))

, nothing more elegant / already existing / ...?

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

Thanks,
Olek