Re: linux-next: tracebacks in workqueue.c/__flush_work()

[Tetsuo Handa]

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@xxxxxxxxxxxxxxxxxxx> 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.