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@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Glauber Costa <glommer@xxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
> 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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