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Date:   Thu, 24 Jun 2021 17:39:02 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Daniel Bristot de Oliveira <bristot@redhat.com>
Cc:     Phil Auld <pauld@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Kate Carcia <kcarcia@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Clark Willaims <williams@redhat.com>,
        John Kacur <jkacur@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V5 12/14] trace: Protect tr->tracing_cpumask with
 get/put_online_cpus
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        <38d2ef13b33c42fcf424a6213a27c8b5246548e0.1624372313.git.bristot@redhat.com>
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On Tue, 22 Jun 2021 16:42:30 +0200
Daniel Bristot de Oliveira <bristot@redhat.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