linux-kernel - Re: [PATCH] x86/mce/therm_throt: Handle case where throttle_active_work() is called on behalf of an offline CPU

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Message-ID: <20200222175151.GD11284@zn.tnic>
Date:   Sat, 22 Feb 2020 18:51:51 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     tony.luck@...el.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, linux-edac@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [PATCH] x86/mce/therm_throt: Handle case where
 throttle_active_work() is called on behalf of an offline CPU

On Sat, Feb 22, 2020 at 08:24:32AM -0800, Srinivas Pandruvada wrote:
> During cpu-hotplug test with CONFIG_PREEMPTION and CONFIG_DEBUG_PREEMPT
> enabled, Chris reported error:
> 
> BUG: using smp_processor_id() in preemptible [00000000] code: kworker/1:0/17
> caller is throttle_active_work+0x12/0x280
> 
> Here throttle_active_work() is a work queue callback scheduled with
> schedule_delayed_work_on(). This will not cause this error for the use
> of smp_processor_id() under normal conditions as there is a check for
> "current->nr_cpus_allowed == 1".
> But when the target CPU is offline the workqueue becomes unbound.
> Then the work queue callback can be scheduled on another CPU and the
> error is printed for the use of smp_processor_id() in preemptible context.

So what's wrong with simply doing:

	if (cpu_is_offline(this_cpu))
		return;

?

You don't need to run the callback on an offlined CPU anyway...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette