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Date:   Wed, 15 Jun 2022 16:11:11 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Muchun Song <songmuchun@bytedance.com>
Subject: Re: [PATCH v2 3/3] mm/kmemleak: Prevent soft lockup in first object
 iteration loop of kmemleak_scan()
Message-ID: <Yqn2jxZhk5Neomu7@arm.com>
References: <20220614220359.59282-1-longman@redhat.com>
 <20220614220359.59282-4-longman@redhat.com>
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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_unlock();
> +			cond_resched();
> +			rcu_read_lock();

cond_resched_rcu() to save a couple of lines?

> +
> +			if (!obj_pinned)
> +				put_object(object);
> +		}

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Thanks.