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

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Message-ID: <20211018220203.064a42ed@gandalf.local.home>
Date:   Mon, 18 Oct 2021 22:02:03 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Petr Mladek <pmladek@...e.com>
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, 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)) {


-- Steve