linux-kernel - Re: linux-next: tracebacks in workqueue.c/__flush

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Message-ID: <87munc306z.fsf@rustcorp.com.au>
Date:   Mon, 04 Feb 2019 10:16:12 +1030
From:   Rusty Russell <rusty@...tcorp.com.au>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Guenter Roeck <linux@...ck-us.net>,
        Chris Metcalf <chris.d.metcalf@...il.com>
Cc:     "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Tejun Heo <tj@...nel.org>, linux-mm <linux-mm@...ck.org>
Subject: Re: linux-next: tracebacks in workqueue.c/__flush_work()

Tetsuo Handa <penguin-kernel@...ove.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_mask if NR_CPUS == 1 ?

No, fix the API to be least-surprise.  Fix 2d3854a37e8b767a too.

Doing anything else would be horrible, IMHO.

Cheers,
Rusty.