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

[Michal Hocko <mhocko@suse.cz>]

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.

> 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
> 

-- 
Michal Hocko
SUSE Labs

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