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

[Vladimir Davydov <vdavydov@parallels.com>]

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]
    <...>

Nothing prevents some archs from moving initialization after setting the
pointer, or reading data before reading the pointer to it.

Of course, I will include a detailed description in the next version of
this patch.

Thanks.

>> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
>> Cc: Michal Hocko <mhocko@suse.cz>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Glauber Costa <glommer@gmail.com>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: Pekka Enberg <penberg@kernel.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> ---
>>  mm/memcontrol.c |   24 ++++++++++--------------
>>  mm/slab.h       |    6 +++++-
>>  2 files changed, 15 insertions(+), 15 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index e6ad6ff..e37fdb5 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3429,12 +3429,14 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
>>  
>>  	atomic_set(&new_cachep->memcg_params->nr_pages , 0);
>>  
>> -	cachep->memcg_params->memcg_caches[idx] = new_cachep;
>>  	/*
>> -	 * the readers won't lock, make sure everybody sees the updated value,
>> -	 * so they won't put stuff in the queue again for no reason
>> +	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
>> +	 * barrier here to ensure nobody will see the kmem_cache partially
>> +	 * initialized.
>>  	 */
>> -	wmb();
>> +	smp_wmb();
>> +
>> +	cachep->memcg_params->memcg_caches[idx] = new_cachep;
>>  out:
>>  	mutex_unlock(&memcg_cache_mutex);
>>  	return new_cachep;
>> @@ -3573,7 +3575,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
>>  					  gfp_t gfp)
>>  {
>>  	struct mem_cgroup *memcg;
>> -	int idx;
>> +	struct kmem_cache *memcg_cachep;
>>  
>>  	VM_BUG_ON(!cachep->memcg_params);
>>  	VM_BUG_ON(!cachep->memcg_params->is_root_cache);
>> @@ -3587,15 +3589,9 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
>>  	if (!memcg_can_account_kmem(memcg))
>>  		goto out;
>>  
>> -	idx = memcg_cache_id(memcg);
>> -
>> -	/*
>> -	 * barrier to mare sure we're always seeing the up to date value.  The
>> -	 * code updating memcg_caches will issue a write barrier to match this.
>> -	 */
>> -	read_barrier_depends();
>> -	if (likely(cache_from_memcg_idx(cachep, idx))) {
>> -		cachep = cache_from_memcg_idx(cachep, idx);
>> +	memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
>> +	if (likely(memcg_cachep)) {
>> +		cachep = memcg_cachep;
>>  		goto out;
>>  	}
>>  
>> diff --git a/mm/slab.h b/mm/slab.h
>> index 0859c42..1d8b53f 100644
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -163,9 +163,13 @@ static inline const char *cache_name(struct kmem_cache *s)
>>  static inline struct kmem_cache *
>>  cache_from_memcg_idx(struct kmem_cache *s, int idx)
>>  {
>> +	struct kmem_cache *cachep;
>> +
>>  	if (!s->memcg_params)
>>  		return NULL;
>> -	return s->memcg_params->memcg_caches[idx];
>> +	cachep = s->memcg_params->memcg_caches[idx];
>> +	smp_read_barrier_depends();	/* see memcg_register_cache() */
>> +	return cachep;
>>  }
>>  
>>  static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
>> -- 
>> 1.7.10.4
>>

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