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

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Message-ID: <YW5ok3CfNoRMfVQ5@alley>
Date:   Tue, 19 Oct 2021 08:41:23 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Colin Ian King <colin.king@...onical.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.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>
Subject: Re: [PATCH] tracing: Have all levels of checks prevent recursion

On Mon 2021-10-18 22:02:03, Steven Rostedt wrote:
> On Mon, 18 Oct 2021 12:19:20 +0200
> Petr Mladek <pmladek@...e.com> wrote:
> 
> > > -
> > >  	bit = trace_get_context_bit() + start;
> > >  	if (unlikely(val & (1 << bit))) {
> > >  		/*
> > >  		 * It could be that preempt_count has not been updated during
> > >  		 * a switch between contexts. Allow for a single recursion.
> > >  		 */
> > > -		bit = TRACE_TRANSITION_BIT;
> > > +		bit = TRACE_CTX_TRANSITION + start;  
> >
> 
> [..]
> 
> > Could we please update the comment? I mean to say if it is a race
> > or if we trace a function that should not get traced.
> 
> What do you think of this change?
> 
> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
> index 1d8cce02c3fb..24f284eb55a7 100644
> --- a/include/linux/trace_recursion.h
> +++ b/include/linux/trace_recursion.h
> @@ -168,8 +168,12 @@ static __always_inline int trace_test_and_set_recursion(unsigned long ip, unsign
>  	bit = trace_get_context_bit() + start;
>  	if (unlikely(val & (1 << bit))) {
>  		/*
> -		 * It could be that preempt_count has not been updated during
> -		 * a switch between contexts. Allow for a single recursion.
> +		 * If an interrupt occurs during a trace, and another trace
> +		 * happens in that interrupt but before the preempt_count is
> +		 * updated to reflect the new interrupt context, then this
> +		 * will think a recursion occurred, and the event will be dropped.
> +		 * Let a single instance happen via the TRANSITION_BIT to
> +		 * not drop those events.
>  		 */
>  		bit = TRACE_TRANSITION_BIT;
>  		if (val & (1 << bit)) {
> 
> 

Looks good to me. Thanks for the update.

Feel free to postpone this change. I do not want to complicate
upstreaming the fix for stable. I am sorry if I already
complicated it.

Best Regards,
Petr