linux-kernel - Re: [PATCH 17/21] Percpu tag allocator

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Date:	Tue, 14 May 2013 15:48:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-aio@...ck.org, akpm@...ux-foundation.org,
	Tejun Heo <tj@...nel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 17/21] Percpu tag allocator

On 05/13, Kent Overstreet 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();
> +			__set_current_state(TASK_RUNNING);

schedule() always returns in TASK_RUNNING state

> +
> +			if (!list_empty_careful(&wait.list)) {
> +				spin_lock_irqsave(&pool->lock, flags);
> +				list_del_init(&wait.list);
> +				spin_unlock_irqrestore(&pool->lock, flags);

This is only theoretical, but racy.

tag_free() does

	list_del_init(wait->list);
	/* WINDOW */
	wake_up_process(wait->task);
	
in theory the caller of tag_alloc() can notice list_empty_careful(),
return without taking pool->lock, exit, and free this task_struct.

But the main problem is that it is not clear why this code reimplements
add_wait_queue/wake_up_all, for what?

I must admit, I do not understand what this code actually does ;)
I didn't try to read it carefully though, but perhaps at least the
changelog could explain more?

Oleg.

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