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SubjectRe: [PATCH v2 3/3] mm/kmemleak: Prevent soft lockup in first object iteration loop of kmemleak_scan()
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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@arm.com>
Thanks,

Longman

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