Re: [PATCH] workqueue: Fix race in schedule and flush work

Padmanabha Srinivasaiah
2022-02-16T18:49:45Z

On Mon, Feb 14, 2022 at 09:43:52AM -1000, Tejun Heo wrote:
> Hello,
>
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 33f1106b4f99..a3f53f859e9d 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -3326,28 +3326,38 @@ EXPORT_SYMBOL(cancel_delayed_work_sync);
> > */
> > int schedule_on_each_cpu(work_func_t func)
> > {
> > - int cpu;
> > struct work_struct __percpu *works;
> > + cpumask_var_t sched_cpumask;
> > + int cpu, ret = 0;
> >
> > - works = alloc_percpu(struct work_struct);
> > - if (!works)
> > + if (!alloc_cpumask_var(&sched_cpumask, GFP_KERNEL))
> > return -ENOMEM;
> >
> > + works = alloc_percpu(struct work_struct);
> > + if (!works) {
> > + ret = -ENOMEM;
> > + goto free_cpumask;
> > + }
> > +
> > cpus_read_lock();
> >
> > - for_each_online_cpu(cpu) {
> > + cpumask_copy(sched_cpumask, cpu_online_mask);
> > + for_each_cpu_and(cpu, sched_cpumask, cpu_online_mask) {
>
> This definitely would need a comment explaining what's going on cuz it looks
> weird to be copying the cpumask which is supposed to stay stable due to the
> cpus_read_lock().Given that it can only happen during early boot and the
> online cpus can only be expanding, maybe just add sth like:
>
> if (early_during_boot) {
> for_each_possible_cpu(cpu)
> INIT_WORK(per_cpu_ptr(works, cpu), func);
> }
>

Thanks tejun for the reply and suggestions.

Yes, unfortunately cpus_read_lock not keeping cpumask stable at
secondary boot. Not sure, may be it only gurantee 'cpu' dont go down
under cpus_read_[lock/unlock].

As suggested will tryout something like:
if (system_state != RUNNING) {
:
}
> BTW, who's calling schedule_on_each_cpu() that early during boot. It makes
> no sense to do this while the cpumasks can't be stabilized.
>
It is implemenation of CONFIG_TASKS_RUDE_RCU.

> Thanks.
>
> --
> tejun