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

[Padmanabha Srinivasaiah <treasure4paddy@gmail.com>]

On Wed, Feb 16, 2022 at 11:07:00AM -0800, Paul E. McKenney wrote:
> On Wed, Feb 16, 2022 at 07:49:39PM +0100, Padmanabha Srinivasaiah wrote:
> > 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.
> 
> Another option would be to adjust CONFIG_TASKS_RUDE_RCU based on where
> things are in the boot process.  For example:
> 
> 	// Wait for one rude RCU-tasks grace period.
> 	static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
> 	{
> 		if (num_online_cpus() <= 1)
> 			return;  // Fastpath for only one CPU.
> 		rtp->n_ipis += cpumask_weight(cpu_online_mask);
> 		schedule_on_each_cpu(rcu_tasks_be_rude);
> 	}
> 
> Easy enough either way!
> 
> 							Thanx, Paul

Thank you Paul for suggestion, tried same and it fixes the issue.
Have submitted same as suggested-by Paul.

Link :
https://lore.kernel.org/all/20220217152520.18972-1-treasure4paddy@gmail.com/T/#t

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