Re: [PATCH 2/2 v5] lib: debugobjects: handle objects free in a batch outside the loop

[Yang Shi]

On 2/1/18 1:36 PM, Thomas Gleixner wrote:
> On Fri, 2 Feb 2018, Yang Shi wrote:
>
> >   /*
> > - * Allocate a new object. If the pool is empty, switch off the debugger.
> > + * Allocate a new object. Retrieve from global freelist first. If the pool is
> > + * empty, switch off the debugger.
> >    * Must be called with interrupts disabled.
> >    */
> >   static struct debug_obj *
> > @@ -150,6 +154,13 @@ static struct debug_obj *lookup_object(void *addr, struct debug_bucket *b)
> >   	struct debug_obj *obj = NULL;
> >   
> >   	raw_spin_lock(&pool_lock);
> Why in alloc_object() and not in fill_pool()?
>
> > +	if (obj_nr_tofree > 0 && (obj_pool_free < obj_pool_min_free)) {
> > +		obj = hlist_entry(obj_to_free.first, typeof(*obj), node);
> > +		obj_nr_tofree--;
> > +		hlist_del(&obj->node);
> > +		goto out;
> > +	}
> Errm. This is wrong. It does not reinitialize the object. Please do that
> shuffling in fill_pool().

OK, will move the reuse logic into fill_pool().

> >   	if (obj_pool.first) {
> >   		obj	    = hlist_entry(obj_pool.first, typeof(*obj), node);
> ....
>
> > +	/* When pool list is not full move free objs to pool list */
> > +	while (obj_pool_free < debug_objects_pool_size) {
> > +		if (obj_nr_tofree <= 0)
> > +			break;
> > +
> > +		obj = hlist_entry(obj_to_free.first, typeof(*obj), node);
> > +		hlist_del(&obj->node);
> > +		hlist_add_head(&obj->node, &obj_pool);
> > +		obj_pool_free++;
> > +		obj_pool_used--;
> > +		obj_nr_tofree--;
> > +	}
> > +
> > +	/*
> > +	 * pool list is already full, and there are still objs on the free list,
> > +	 * move remaining free objs to a separate list to free the memory later.
> > +	 */
> > +	if (obj_nr_tofree > 0) {
> > +		hlist_move_list(&obj_to_free, &tofree);
> > +		obj_nr_tofree = 0;
> > +	}
> The accounting is inconsistent. You leak obj_pool_used. But that's wrong
> anyway because an object should not be accounted for in two places. It's
> only on _ONE_ list....

So I should move the accounting to where the obj is deleted from the 
list? It should look like:

if (obj_nr_tofree > 0)
        hlist_move_list(&obj_to_free, &tofree);

...

if (!hlist_empty(&tofree)) {
                hlist_for_each_entry_safe(obj, tmp, &tofree, node) {
                        hlist_del(&obj->node);
                        obj_nr_tofree--;
                        kmem_cache_free(obj_cache, obj);
                }
        }

> > @@ -716,7 +762,6 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size)
> >   {
> >   	unsigned long flags, oaddr, saddr, eaddr, paddr, chunks;
> >   	struct hlist_node *tmp;
> > -	HLIST_HEAD(freelist);
> >   	struct debug_obj_descr *descr;
> >   	enum debug_obj_state state;
> >   	struct debug_bucket *db;
> > @@ -752,18 +797,17 @@ static void __debug_check_no_obj_freed(const void *address, unsigned long size)
> >   				goto repeat;
> >   			default:
> >   				hlist_del(&obj->node);
> > -				hlist_add_head(&obj->node, &freelist);
> > +				/* Put obj on the global free list */
> > +				raw_spin_lock(&pool_lock);
> > +				hlist_add_head(&obj->node, &obj_to_free);
> > +				/* Update the counter of objs on the global freelist */
> > +				obj_nr_tofree++;
> > +				raw_spin_unlock(&pool_lock);
> As we have to take pool_lock anyway, we simply can change free_object() to:
>
> static bool __free_object(obj)
> {
> 	bool work;
>
> 	lock(pool);
> 	work = obj_pool_free > debug_objects_pool_size && obj_cache;
> 	obj_pool_used++;
> 	if (work) {
> 		obj_nr_tofree++;
> 		hlist_add_head(&obj->node, &obj_to_free);
> 	] else {
> 		obj_pool_free++;
> 		hlist_add_head(&obj->node, &obj_pool);
> 	}
> 	unlock(pool);
> 	return work;
> }
>
> static void free_object(obj)
> {
> 	if (__free_object(obj))
> 		schedule_work(&debug_obj_work);
> }
>
> and then use __free_object() in __debug_check_no_obj_freed()
>
>      	 bool work = false;
>
> 	 ...
>   	    	 work |= __free_object(obj);
> 	 ...
>
> 	 if (work)
> 		schedule_work(&debug_obj_work);
>
> That makes the whole thing simpler and the accounting is matching the place
> where the object is:
>
>        obj_pool_free counts the number of objects enqueued in obj_pool
>        obj_nr_tofree counts the number of objects enqueued in obj_to_free
>        obr_pool_used counts the number of objects enqueued in the hash lists
>
> Ideally you split that patch into pieces:
>
> 1) Introduce obj_to_free/obj_nr_tofree and add the removing/freeing from it
>     in fill_pool() and free_obj_work(). Nothing adds an object at this point
>     to obj_to_free.
>
> 2) Change free_object() to use obj_to_free and split it apart
>
> 3) Change __debug_check_no_obj_freed() to use __free_object()
>
> That makes it simpler to review and to follow.
>
> Hmm?

Sure, will refactor free_object() and split the patches in newer version.

Thanks,
Yang

> Thanks,
>
> 	tglx