linux-kernel - Re: [PATCH AUTOSEL 4.9 4/4] thermal: intel_powerclamp: Use get_cpu() instead of smp_processor

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Date:   Tue, 11 Oct 2022 13:36:46 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Chen Yu <yu.c.chen@...el.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        rafael@...nel.org, daniel.lezcano@...aro.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 4.9 4/4] thermal: intel_powerclamp: Use get_cpu()
 instead of smp_processor_id() to avoid crash

Hi!

> From: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> 
> [ Upstream commit 68b99e94a4a2db6ba9b31fe0485e057b9354a640 ]
> 
> When CPU 0 is offline and intel_powerclamp is used to inject
> idle, it generates kernel BUG:
> 
> BUG: using smp_processor_id() in preemptible [00000000] code: bash/15687
> caller is debug_smp_processor_id+0x17/0x20
> CPU: 4 PID: 15687 Comm: bash Not tainted 5.19.0-rc7+ #57
> Call Trace:
> <TASK>
> dump_stack_lvl+0x49/0x63
> dump_stack+0x10/0x16
> check_preemption_disabled+0xdd/0xe0
> debug_smp_processor_id+0x17/0x20
> powerclamp_set_cur_state+0x7f/0xf9 [intel_powerclamp]
> ...
> ...
> 
> Here CPU 0 is the control CPU by default and changed to the current CPU,
> if CPU 0 offlined. This check has to be performed under cpus_read_lock(),
> hence the above warning.
> 
> Use get_cpu() instead of smp_processor_id() to avoid this BUG.

This has exactly the same problem as smp_processor_id(), you just
worked around the warning. If it is okay that control_cpu contains
stale value, could we have a comment explaining why?

Thanks,
								Pavel
								
> +++ b/drivers/thermal/intel_powerclamp.c
> @@ -519,8 +519,10 @@ static int start_power_clamp(void)
>  
>  	/* prefer BSP */
>  	control_cpu = 0;
> -	if (!cpu_online(control_cpu))
> -		control_cpu = smp_processor_id();
> +	if (!cpu_online(control_cpu)) {
> +		control_cpu = get_cpu();
> +		put_cpu();
> +	}
>  
>  	clamping = true;
>  	schedule_delayed_work(&poll_pkg_cstate_work, 0);
> -- 
> 2.35.1

-- 
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