linux-kernel - Re: [PATCH 2/2] ftrace: prevent preemption in perf_ftrace_function

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Message-ID: <YWVvfBybqjKuifum@hirez.programming.kicks-ass.net>
Date:   Tue, 12 Oct 2021 13:20:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     王贇 <yun.wang@...ux.alibaba.com>
Cc:     Guo Ren <guoren@...nel.org>, Steven Rostedt <rostedt@...dmis.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-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        live-patching@...r.kernel.org
Subject: Re: [PATCH 2/2] ftrace: prevent preemption in
 perf_ftrace_function_call()

On Tue, Oct 12, 2021 at 01:40:31PM +0800, 王贇 wrote:

> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index 6aed10e..33c2f76 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -441,12 +441,19 @@ void perf_trace_buf_update(void *record, u16 type)
>  	if (!rcu_is_watching())
>  		return;
> 
> +	/*
> +	 * Prevent CPU changing from now on. rcu must
> +	 * be in watching if the task was migrated and
> +	 * scheduled.
> +	 */
> +	preempt_disable_notrace();
> +
>  	if ((unsigned long)ops->private != smp_processor_id())
> -		return;
> +		goto out;
> 
>  	bit = ftrace_test_recursion_trylock(ip, parent_ip);
>  	if (bit < 0)
> -		return;
> +		goto out;
> 
>  	event = container_of(ops, struct perf_event, ftrace_ops);
> 

This seems rather daft, wouldn't it be easier to just put that check
under the recursion thing?