Re: [PATCH v3 1/3] lib/percpu-list: Per-cpu list with associated per-cpu locks

[Waiman Long]

On 02/24/2016 02:56 AM, Jan Kara wrote:
> On Tue 23-02-16 14:04:30, Waiman Long wrote:
> > Linked list is used everywhere in the Linux kernel. However, if many
> > threads are trying to add or delete entries into the same linked list,
> > it can create a performance bottleneck.
> >
> > This patch introduces a new per-cpu list subystem with associated
> > per-cpu locks for protecting each of the lists individually. This
> > allows list entries insertion and deletion operations to happen in
> > parallel instead of being serialized with a global list and lock.
> >
> > List entry insertion is strictly per cpu. List deletion, however, can
> > happen in a cpu other than the one that did the insertion. So we still
> > need lock to protect the list. Because of that, there may still be
> > a small amount of contention when deletion is being done.
> >
> > A new header file include/linux/percpu-list.h will be added with the
> > associated pcpu_list_head and pcpu_list_node structures. The following
> > functions are provided to manage the per-cpu list:
> >
> >   1. int init_pcpu_list_head(struct pcpu_list_head **ppcpu_head)
> >   2. void pcpu_list_add(struct pcpu_list_node *node,
> > 		       struct pcpu_list_head *head)
> >   3. void pcpu_list_del(struct pcpu_list *node)
> >
> > Iteration of all the list entries within a group of per-cpu
> > lists is done by calling either the pcpu_list_iterate() or
> > pcpu_list_iterate_safe() functions in a while loop. They correspond
> > to the list_for_each_entry() and list_for_each_entry_safe() macros
> > respectively. The iteration states are keep in a pcpu_list_state
> > structure that is passed to the iteration functions.
> >
> > Signed-off-by: Waiman Long<Waiman.Long@xxxxxxx>
> Two comments below.
>
> > +/*
> > + * Helper function to find the first entry of the next per-cpu list
> > + * It works somewhat like for_each_possible_cpu(cpu).
> > + *
> > + * Return: true if the entry is found, false if all the lists exhausted
> > + */
> > +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;
> > +	state->lock =&per_cpu_ptr(head, state->cpu)->lock;
> > +	state->curr = list_entry(state->head->next,
> > +				 struct pcpu_list_node, list);
> > +	if (&state->curr->list == state->head)
> > +		goto next_cpu;
> This might be more comprehensible as:
>
> 	if (list_empty(state->head))
> 		goto next_cpu;
>
> and you can do it just after updating state->head (no need to init
> state->lock&  state->curr if the list is empty).

Thank for the suggestion. Will change the code accordingly.
> Another note: Initialization of state->curr is IMO racy - you need to hold
> state->lock to grab state->curr reliably, don't you? Otherwise someone can
> remove the entry while you are working with it. So you need to move that
> down just before returning.

Right. I will move the initialization of state->curr after the spin_lock().

> > +
> > +	spin_lock(state->lock);
> > +	return true;
> > +}
> > +#endif /* NR_CPUS == 1 */
> ...
>
> > +/*
> > + * Delete a node from a percpu list
> > + *
> > + * We need to check the lock pointer again after taking the lock to guard
> > + * against concurrent delete of the same node. If the lock pointer changes
> > + * (becomes NULL or to a different one), we assume that the deletion was done
> > + * elsewhere.
> > + */
> > +void pcpu_list_del(struct pcpu_list_node *node)
> > +{
> > +	spinlock_t *lock = READ_ONCE(node->lockptr);
> > +
> > +	if (unlikely(!lock))
> > +		return;
> > +
> > +	spin_lock(lock);
> > +	if (likely(lock == node->lockptr)) {
> > +		list_del_init(&node->list);
> > +		node->lockptr = NULL;
> > +	}
> But someone changing lockptr under your hands would mean that there are
> two processes racing to remove entries and that would generally point to a
> problem (and likely use-after-free) in the caller, won't it? Or do you have
> some particular usecase in mind?
>
> 								Honza

This is just defensive programming to guard against unforeseen case. I 
don't have any particular use case in mind that will make that happen. 
Maybe I should put a WARN_ON if this really happens.

Cheers,
Longman