linux-kernel - Re: [PATCH v2 3/3] mm/kmemleak: Prevent soft lockup in first object iteration loop of kmemleak

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Date:   Wed, 15 Jun 2022 12:07:34 -0400
From:   Waiman Long <longman@...hat.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v2 3/3] mm/kmemleak: Prevent soft lockup in first object
 iteration loop of kmemleak_scan()

On 6/15/22 11:11, Catalin Marinas wrote:
> On Tue, Jun 14, 2022 at 06:03:59PM -0400, Waiman Long wrote:
>> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
>> index 7dd64139a7c7..abba063ae5ee 100644
>> --- a/mm/kmemleak.c
>> +++ b/mm/kmemleak.c
>> @@ -1417,12 +1417,16 @@ static void kmemleak_scan(void)
>>   	struct zone *zone;
>>   	int __maybe_unused i;
>>   	int new_leaks = 0;
>> +	int loop1_cnt = 0;
>>   
>>   	jiffies_last_scan = jiffies;
>>   
>>   	/* prepare the kmemleak_object's */
>>   	rcu_read_lock();
>>   	list_for_each_entry_rcu(object, &object_list, object_list) {
>> +		bool obj_pinned = false;
>> +
>> +		loop1_cnt++;
>>   		raw_spin_lock_irq(&object->lock);
>>   #ifdef DEBUG
>>   		/*
>> @@ -1437,10 +1441,32 @@ static void kmemleak_scan(void)
>>   #endif
>>   		/* reset the reference count (whiten the object) */
>>   		object->count = 0;
>> -		if (color_gray(object) && get_object(object))
>> +		if (color_gray(object) && get_object(object)) {
>>   			list_add_tail(&object->gray_list, &gray_list);
>> +			obj_pinned = true;
>> +		}
>>   
>>   		raw_spin_unlock_irq(&object->lock);
>> +
>> +		/*
>> +		 * Do a cond_resched() to avoid soft lockup every 64k objects.
>> +		 * Make sure a reference has been taken so that the object
>> +		 * won't go away without RCU read lock.
>> +		 */
>> +		if (!(loop1_cnt & 0xffff)) {
>> +			if (!obj_pinned && !get_object(object)) {
>> +				/* Try the next object instead */
>> +				loop1_cnt--;
>> +				continue;
>> +			}
> With this trick we could probably get rid of rcu_read_lock() and take
> the kmemleak_lock instead. But that's for another patch.
rcu_read_lock() is cheap unless we use a preempt kernel.

>
>> +
>> +			rcu_read_unlock();
>> +			cond_resched();
>> +			rcu_read_lock();
> cond_resched_rcu() to save a couple of lines?
I am not aware of such helper function. Yes, I should have used that 
instead in case I need to update this patch again. Thanks!
>
>> +
>> +			if (!obj_pinned)
>> +				put_object(object);
>> +		}
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>
Thanks,

Longman