Re: [PATCH v2 3/3] mm/kmemleak: Prevent soft lockup in first object iteration loop of kmemleak_scan()

[Waiman Long]

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@xxxxxxx>
Thanks,

Longman