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

From: Borislav Petkov
Date: Sat Feb 22 2020 - 12:52:06 EST

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))


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