linux-kernel - Re: [PATCH 2/4 v6] lib: debugobjects: add global free list and the counter

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Date:   Mon, 12 Feb 2018 17:25:36 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
cc:     longman@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4 v6] lib: debugobjects: add global free list and the
 counter

On Tue, 6 Feb 2018, Yang Shi wrote:
> +	/*
> +	 * Reuse objs from the global free list, they will be reinitialized
> +	 * when allocating
> +	 */
> +	while (obj_nr_tofree > 0 && (obj_pool_free < obj_pool_min_free)) {
> +		raw_spin_lock_irqsave(&pool_lock, flags);
> +		obj = hlist_entry(obj_to_free.first, typeof(*obj), node);

This is racy vs. the worker thread. Assume obj_nr_tofree = 1:

CPU0					CPU1
worker
   lock(&pool_lock);			while (obj_nr_tofree > 0 && ...) {
     obj = hlist_entry(obj_to_free);	  lock(&pool_lock);
     hlist_del(obj);			  
     obj_nr_tofree--;
     ...
   unlock(&pool_lock);
					  obj = hlist_entry(obj_to_free);
					  hlist_del(obj); <------- NULL pointer dereference

Not what you want, right? The counter or the list head need to be rechecked
after the lock is acquired.

> +		hlist_del(&obj->node);
> +		obj_nr_tofree--;
> +		hlist_add_head(&obj->node, &obj_pool);
> +		obj_pool_free++;
> +		raw_spin_unlock_irqrestore(&pool_lock, flags);
> +	}

Thanks,

	tglx