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 10:48:44 -0800
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 2/12/18 8:25 AM, Thomas Gleixner wrote:
> 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.

Yes, you are right. Will fix the race in newer version.

Regards,
Yang

>
>> +		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