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Date:   Wed, 6 Feb 2019 08:23:59 -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: <20190206162359.GA30699@roeck-us.net>
References: <72e7d782-85f2-b499-8614-9e3498106569@i-love.sakura.ne.jp>
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On Wed, Feb 06, 2019 at 11:57:45PM +0900, Tetsuo Handa wrote:
> On 2019/02/06 23:36, Guenter Roeck wrote:
> > 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.
> 
> Fixing macros is fine, The problem is that "mask" becomes evaluated
> which might be currently undefined or unassigned if CONFIG_SMP=n.
> Evaluating "mask" generates expected behavior for lru_add_drain_all()
> case. But there might be cases where evaluating "mask" generate
> unexpected behavior/results.

Interesting notion. I would have assumed that passing a parameter
to a function or macro implies that this parameter may be used.

This makes me wonder - what is the point of ", (mask)" in the current
macros ? It doesn't make sense to me.

Anyway, I agree that fixing the macro might result in some failures.
However, I would argue that those failures would actually be bugs,
hidden by the buggy macros. But of course that it just my opinion.

Guenter