linux-kernel - Re: [PATCH V5 12/14] trace: Protect tr->tracing_cpumask with get/put_online

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Message-ID: <20210624173902.57f4f34f@oasis.local.home>
Date:   Thu, 24 Jun 2021 17:39:02 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     Phil Auld <pauld@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Kate Carcia <kcarcia@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Clark Willaims <williams@...hat.com>,
        John Kacur <jkacur@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 12/14] trace: Protect tr->tracing_cpumask with
 get/put_online_cpus

On Tue, 22 Jun 2021 16:42:30 +0200
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:

> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 52fc9438b7b4..c14f33db147e 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -5053,7 +5053,13 @@ int tracing_set_cpumask(struct trace_array *tr,
>  	arch_spin_unlock(&tr->max_lock);
>  	local_irq_enable();
>  
> +	/*
> +	 * tracing_cpumask is read by tracers that support CPU
> +	 * hotplug.
> +	 */
> +	get_online_cpus();
>  	cpumask_copy(tr->tracing_cpumask, tracing_cpumask_new);
> +	put_online_cpus();
>  
>  	return 0;

Hmm, the tracing_cpumask is only touched in he work function, with the
necessary locks. How is get_online_cpus() protecting it here?

That is, tracing_cpumask isn't touched in the path of bringing up or
taking down a CPU, and shouldn't be an issue here.

Should I just drop this patch?

-- Steve