On Tue, May 28, 2019 at 01:37:50PM -0400, Waiman Long wrote:
> On 5/28/19 1:08 PM, Vladimir Davydov wrote:
> >> static void flush_memcg_workqueue(struct kmem_cache *s)
> >> {
> >> + /*
> >> + * memcg_params.dying is synchronized using slab_mutex AND
> >> + * memcg_kmem_wq_lock spinlock, because it's not always
> >> + * possible to grab slab_mutex.
> >> + */
> >> mutex_lock(&slab_mutex);
> >> + spin_lock(&memcg_kmem_wq_lock);
> >> s->memcg_params.dying = true;
> >> + spin_unlock(&memcg_kmem_wq_lock);
> > I would completely switch from the mutex to the new spin lock -
> > acquiring them both looks weird.
> >
> >> mutex_unlock(&slab_mutex);
> >>
> >> /*
>
> There are places where the slab_mutex is held and sleeping functions
> like kvzalloc() are called. I understand that taking both mutex and
> spinlocks look ugly, but converting all the slab_mutex critical sections
> to spinlock critical sections will be a major undertaking by itself. So
> I would suggest leaving that for now.
I didn't mean that. I meant taking spin_lock wherever we need to access
the 'dying' flag, even if slab_mutex is held. So that we don't need to
take mutex_lock in flush_memcg_workqueue, where it's used solely for
'dying' synchronization.
On 5/28/19 1:39 PM, Vladimir Davydov wrote:
> On Tue, May 28, 2019 at 01:37:50PM -0400, Waiman Long wrote:
>> On 5/28/19 1:08 PM, Vladimir Davydov wrote:
>>>> static void flush_memcg_workqueue(struct kmem_cache *s)
>>>> {
>>>> + /*
>>>> + * memcg_params.dying is synchronized using slab_mutex AND
>>>> + * memcg_kmem_wq_lock spinlock, because it's not always
>>>> + * possible to grab slab_mutex.
>>>> + */
>>>> mutex_lock(&slab_mutex);
>>>> + spin_lock(&memcg_kmem_wq_lock);
>>>> s->memcg_params.dying = true;
>>>> + spin_unlock(&memcg_kmem_wq_lock);
>>> I would completely switch from the mutex to the new spin lock -
>>> acquiring them both looks weird.
>>>
>>>> mutex_unlock(&slab_mutex);
>>>>
>>>> /*
>> There are places where the slab_mutex is held and sleeping functions
>> like kvzalloc() are called. I understand that taking both mutex and
>> spinlocks look ugly, but converting all the slab_mutex critical sections
>> to spinlock critical sections will be a major undertaking by itself. So
>> I would suggest leaving that for now.
> I didn't mean that. I meant taking spin_lock wherever we need to access
> the 'dying' flag, even if slab_mutex is held. So that we don't need to
> take mutex_lock in flush_memcg_workqueue, where it's used solely for
> 'dying' synchronization.
OK, that makes sense. Thanks for the clarification.
Cheers,
Longman