linux-kernel - Re: [PATCH v5 5/7] mm: rework non-root kmem

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Date:   Tue, 28 May 2019 13:41:34 -0400
From:   Waiman Long <longman@...hat.com>
To:     Vladimir Davydov <vdavydov.dev@...il.com>
Cc:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Christoph Lameter <cl@...ux.com>, cgroups@...r.kernel.org
Subject: Re: [PATCH v5 5/7] mm: rework non-root kmem_cache lifecycle
 management

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