linux-kernel - Re: [PATCH 4/4] kthread: Simplify kthread

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Message-ID: <20180612124257.GS12198@hirez.programming.kicks-ass.net>
Date:   Tue, 12 Jun 2018 14:42:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     mingo@...nel.org, gkohli@...eaurora.org, tglx@...utronix.de,
        mpe@...erman.id.au, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, will.deacon@....com
Subject: Re: [PATCH 4/4] kthread: Simplify kthread_park() completion

On Fri, Jun 08, 2018 at 11:52:20AM +0200, Oleg Nesterov wrote:
> in smpboot_update_cpumask_percpu_thread() can hit a KTHREAD_SHOULD_PARK
> thread? Lets look into kernel test robot's .config:
> 
> 	CONFIG_NR_CPUS=1
> 
> Now look at NR_CPUS==1 version of for_each_cpu* helpers:
> 
> 	#define for_each_cpu(cpu, mask)			\
> 		for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)

Argh, that issue again.

> So even if the "tmp" cpumask is empty the code above always does
> 
> 		smpboot_park_thread(plug_thread, 0);
> 
> and hits the already parked kthread.

OK, I'll write a new Changelog.