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

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Date:   Fri, 21 Jul 2023 12:00:40 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexander Lobakin <aleksander.lobakin@...el.com>
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

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

> From: Steven Rostedt <rostedt@...dmis.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@...dmis.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

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.

-- Steve


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