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From: Thomas Gleixner <tglx@kernel.org>
To: Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>, Johannes
 Weiner <hannes@cmpxchg.org>, Michal =?utf-8?Q?Koutn=C3=BD?=
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Cc: cgroups@vger.kernel.org, linux-doc@vger.kernel.org,
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 linux-kselftest@vger.kernel.org, Costa Shulyupin <cshulyup@redhat.com>,
 Qiliang Yuan <realwujing@gmail.com>, Waiman Long <longman@redhat.com>
Subject: Re: [PATCH 10/23] cpu: Use RCU to protect access of HK_TYPE_TIMER
 cpumask
In-Reply-To: <20260421030351.281436-11-longman@redhat.com>
References: <20260421030351.281436-1-longman@redhat.com>
 <20260421030351.281436-11-longman@redhat.com>
Date: Tue, 21 Apr 2026 10:57:58 +0200
Message-ID: <87wly0bsh5.ffs@tglx>
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On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
> As HK_TYPE_TIMER cpumask is going to be changeable at run time, use
> RCU to protect access to the cpumask.
>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cpu.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bc4f7a9ba64e..0d02b5d7a7ba 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1890,6 +1890,8 @@ int freeze_secondary_cpus(int primary)
>  	cpu_maps_update_begin();
>  	if (primary == -1) {
>  		primary = cpumask_first(cpu_online_mask);
> +
> +		guard(rcu)();
>  		if (!housekeeping_cpu(primary, HK_TYPE_TIMER))
>  			primary = housekeeping_any_cpu(HK_TYPE_TIMER);

housekeeping_cpu() and housekeeping_any_cpu() can operate on two
different CPU masks once the runtime update is enabled.

Seriously?