Re: [PATCH 3/3] mm/kmemleak: Prevent soft lockup in first object iteration loop of kmemleak_scan()

[Catalin Marinas <catalin.marinas@arm.com>]

On Sun, Jun 12, 2022 at 02:33:01PM -0400, Waiman Long wrote:
> @@ -1437,10 +1440,25 @@ 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);
> +			gray_list_cnt++;
> +			object_pinned = true;
> +		}
>  
>  		raw_spin_unlock_irq(&object->lock);
> +
> +		/*
> +		 * With object pinned by a positive reference count, it
> +		 * won't go away and we can safely release the RCU read
> +		 * lock and do a cond_resched() to avoid soft lockup every
> +		 * 64k objects.
> +		 */
> +		if (object_pinned && !(gray_list_cnt & 0xffff)) {
> +			rcu_read_unlock();
> +			cond_resched();
> +			rcu_read_lock();
> +		}

I'm not sure this gains much. There should be very few gray objects
initially (those passed to kmemleak_not_leak() for example). The
majority should be white objects.

If we drop the fine-grained object->lock, we could instead take
kmemleak_lock outside the loop with a cond_resched_lock(&kmemleak_lock)
within the loop. I think we can get away with not having an
rcu_read_lock() at all for list traversal with the big lock outside the
loop.

The reason I added it in the first kmemleak incarnation was to defer
kmemleak_object freeing as it was causing a re-entrant call into the
slab allocator. I later went for fine-grained locking and RCU list
traversal but I may have overdone it ;).

-- 
Catalin