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

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Date:   Wed, 16 Feb 2022 19:49:39 +0100
From:   Padmanabha Srinivasaiah <treasure4paddy@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
        paulmck@...nel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] workqueue: Fix race in schedule and flush work

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