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Date:   Wed, 6 Feb 2019 06:36:25 -0800
From:   Guenter Roeck <linux@roeck-us.net>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Rusty Russell <rusty@rustcorp.com.au>,
        Chris Metcalf <chris.d.metcalf@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>, linux-mm <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>
Subject: Re: linux-next: tracebacks in workqueue.c/__flush_work()
Message-ID: <20190206143625.GA25998@roeck-us.net>
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 <87munc306z.fsf@rustcorp.com.au>
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On Wed, Feb 06, 2019 at 03:31:09PM +0900, Tetsuo Handa wrote:
> (Adding linux-arch ML.)
> 
> Rusty Russell wrote:
> > Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> writes:
> > > (Adding Chris Metcalf and Rusty Russell.)
> > >
> > > If NR_CPUS == 1 due to CONFIG_SMP=n, for_each_cpu(cpu, &has_work) loop does not
> > > evaluate "struct cpumask has_work" modified by cpumask_set_cpu(cpu, &has_work) at
> > > previous for_each_online_cpu() loop. Guenter Roeck found a problem among three
> > > commits listed below.
> > >
> > >   Commit 5fbc461636c32efd ("mm: make lru_add_drain_all() selective")
> > >   expects that has_work is evaluated by for_each_cpu().
> > >
> > >   Commit 2d3854a37e8b767a ("cpumask: introduce new API, without changing anything")
> > >   assumes that for_each_cpu() does not need to evaluate has_work.
> > >
> > >   Commit 4d43d395fed12463 ("workqueue: Try to catch flush_work() without INIT_WORK().")
> > >   expects that has_work is evaluated by for_each_cpu().
> > >
> > > What should we do? Do we explicitly evaluate has_work if NR_CPUS == 1 ?
> > 
> > No, fix the API to be least-surprise.  Fix 2d3854a37e8b767a too.
> > 
> > Doing anything else would be horrible, IMHO.
> > 
> 
> Fixing 2d3854a37e8b767a might involve subtle changes. If we do
> 

Why not fix the macros ?

#define for_each_cpu(cpu, mask)                 \
        for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)

does not really make sense since it does not evaluate mask.

#define for_each_cpu(cpu, mask)                 \
        for ((cpu) = 0; (cpu) < 1 && cpumask_test_cpu((cpu), (mask)); (cpu)++)

or something similar might do it.

Guenter