Re: [PATCH v6 1/4] lib/percpu-list: Per-cpu list with associated per-cpu locks

[Waiman Long]

On 03/21/2016 05:49 AM, Jan Kara wrote:
> On Fri 18-03-16 15:44:01, Waiman Long wrote:
> > +static __always_inline bool
> > +__pcpu_list_next_cpu(struct pcpu_list_head *head, struct pcpu_list_state *state)
> > +{
> > +	if (state->lock)
> > +		spin_unlock(state->lock);
> > +next_cpu:
> > +	/*
> > +	 * for_each_possible_cpu(cpu)
> > +	 */
> > +	state->cpu = cpumask_next(state->cpu, cpu_possible_mask);
> > +	if (state->cpu>= nr_cpu_ids)
> > +		return false;	/* All the per-cpu lists iterated */
> > +
> > +	state->head =&per_cpu_ptr(head, state->cpu)->list;
> > +	if (list_empty(state->head))
> > +		goto next_cpu;
> > +
> > +	state->lock =&per_cpu_ptr(head, state->cpu)->lock;
> > +	spin_lock(state->lock);
> > +	state->curr = list_entry(state->head->next,
> > +				 struct pcpu_list_node, list);
> > +	return true;
> Waiman, I repeat it for the third time as you keep ignoring it: This is
> *racy*. The list for state->cpu can be empty by the time you acquire
> state->lock and thus state->curr will point somewhere around the head of
> the list but definitely not to a list member where it should.
>
> 								Honza

I am sorry for missing your previous comment. Yes, it is possible that 
the list is empty after the lock. So I should have checked for that 
before returning. Thanks for reminding me that. I will fix that later on.

Cheers,
Longman