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

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