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

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Date:   Tue, 13 Feb 2018 21:33:25 -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 2/13/18 2:02 AM, Thomas Gleixner wrote:
> On Mon, 12 Feb 2018, Yang Shi wrote:
>> 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.
> I fixed up all the minor issues with this series and applied it to:
>
>     git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git core/debugobjects
>
> Please double check the result.

Thanks a lot. It looks good.

Regards,
Yang

>
> Thanks,
>
> 	tglx