Re: [PATCH 1/3] Percpu tag allocator

[Oleg Nesterov <oleg@redhat.com>]

On 06/08, Nicholas A. Bellinger wrote:
>
> +unsigned tag_alloc(struct tag_pool *pool, bool wait)
> +{
> +	struct tag_cpu_freelist *tags;
> +	unsigned long flags;
> +	unsigned ret;
> +retry:
> +	preempt_disable();
> +	local_irq_save(flags);
> +	tags = this_cpu_ptr(pool->tag_cpu);
> +
> +	while (!tags->nr_free) {
> +		spin_lock(&pool->lock);
> +
> +		if (pool->nr_free)
> +			move_tags(tags->free, &tags->nr_free,
> +				  pool->free, &pool->nr_free,
> +				  min(pool->nr_free, pool->watermark));
> +		else if (wait) {
> +			struct tag_waiter wait = { .task = current };
> +
> +			__set_current_state(TASK_UNINTERRUPTIBLE);
> +			list_add(&wait.list, &pool->wait);
> +
> +			spin_unlock(&pool->lock);
> +			local_irq_restore(flags);
> +			preempt_enable();
> +
> +			schedule();
> +
> +			if (!list_empty_careful(&wait.list)) {
> +				spin_lock_irqsave(&pool->lock, flags);
> +				list_del_init(&wait.list);
> +				spin_unlock_irqrestore(&pool->lock, flags);
> +			}
> +
> +			goto retry;
> +		} else
> +			goto fail;
> +
> +		spin_unlock(&pool->lock);
> +	}
> +
> +	ret = tags->free[--tags->nr_free];
> +
> +	local_irq_restore(flags);
> +	preempt_enable();
> +
> +	return ret;
> +fail:
> +	local_irq_restore(flags);
> +	preempt_enable();
> +	return 0;
> +}

I still think this code should use the normal wait_event().

See http://marc.info/?l=linux-kernel&m=136863269729888

> +void tag_free(struct tag_pool *pool, unsigned tag)
> +{
> +	struct tag_cpu_freelist *tags;
> +	unsigned long flags;
> +
> +	preempt_disable();
> +	local_irq_save(flags);
> +	tags = this_cpu_ptr(pool->tag_cpu);
> +
> +	tags->free[tags->nr_free++] = tag;
> +
> +	if (tags->nr_free == pool->watermark * 2) {
> +		spin_lock(&pool->lock);
> +
> +		move_tags(pool->free, &pool->nr_free,
> +			  tags->free, &tags->nr_free,
> +			  pool->watermark);
> +
> +		while (!list_empty(&pool->wait)) {
> +			struct tag_waiter *wait;
> +			wait = list_first_entry(&pool->wait,
> +						struct tag_waiter, list);
> +			list_del_init(&wait->list);
> +			wake_up_process(wait->task);

And this still looks racy.
see http://marc.info/?l=linux-kernel&m=136853955229504

And probably the changelog should mention that cpu_down() can
lose the tags.

Oleg.