Re: [PATCH 3/6] memcg, slab: cleanup barrier usage when accessing memcg_caches

[Vladimir Davydov <vdavydov@parallels.com>]

On 12/19/2013 01:21 PM, Michal Hocko wrote:
> On Thu 19-12-13 13:16:01, Vladimir Davydov wrote:
>> On 12/19/2013 01:10 PM, Michal Hocko wrote:
>>> On Thu 19-12-13 10:37:27, Vladimir Davydov wrote:
>>>> On 12/18/2013 09:14 PM, Michal Hocko wrote:
>>>>> On Wed 18-12-13 17:16:54, Vladimir Davydov wrote:
>>>>>> First, in memcg_create_kmem_cache() we should issue the write barrier
>>>>>> after the kmem_cache is initialized, but before storing the pointer to
>>>>>> it in its parent's memcg_params.
>>>>>>
>>>>>> Second, we should always issue the read barrier after
>>>>>> cache_from_memcg_idx() to conform with the write barrier.
>>>>>>
>>>>>> Third, its better to use smp_* versions of barriers, because we don't
>>>>>> need them on UP systems.
>>>>> Please be (much) more verbose on Why. Barriers are tricky and should be
>>>>> documented accordingly. So if you say that we should issue a barrier
>>>>> always be specific why we should do it.
>>>> In short, we have kmem_cache::memcg_params::memcg_caches is an array of
>>>> pointers to per-memcg caches. We access it lock-free so we should use
>>>> memory barriers during initialization. Obviously we should place a write
>>>> barrier just before we set the pointer in order to make sure nobody will
>>>> see a partially initialized structure. Besides there must be a read
>>>> barrier between reading the pointer and accessing the structure, to
>>>> conform with the write barrier. It's all that similar to rcu_assign and
>>>> rcu_deref. Currently the barrier usage looks rather strange:
>>>>
>>>> memcg_create_kmem_cache:
>>>>     initialize kmem
>>>>     set the pointer in memcg_caches
>>>>     wmb() // ???
>>>>
>>>> __memcg_kmem_get_cache:
>>>>     <...>
>>>>     read_barrier_depends() // ???
>>>>     cachep = root_cache->memcg_params->memcg_caches[memcg_id]
>>>>     <...>
>>> Why do we need explicit memory barriers when we can use RCU?
>>> __memcg_kmem_get_cache already dereferences within rcu_read_lock.
>> Because it's not RCU, IMO. RCU implies freeing the old version after a
>> grace period, while kmem_caches are freed immediately. We simply want to
>> be sure the kmem_cache is fully initialized. And we do not require
>> calling this in an RCU critical section.
> And you can use rcu_dereference and rcu_assign for that as well.

rcu_dereference() will complain if called outside an RCU critical
section, while cache_from_memcg_idx() is called w/o RCU protection from
some places.

> It hides all the juicy details about memory barriers.

IMO, a memory barrier with a good comment looks better than an
rcu_dereference() without RCU protection :-)

> Besides that nothing prevents us from freeing from rcu callback. Or?

It's an overhead we can live without there. The point is that we can
access a cache only if it is active. I mean no allocation can go from a
cache that has already been destroyed. It would be a bug. So there is no
point in introducing RCU-protection for kmem_caches there. It would only
confuse, IMO.

Thanks.

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