memcg, slab: fix barrier usage when accessing memcg_caches
Each root kmem_cache has pointers to per-memcg caches stored in its memcg_params::memcg_caches array. Whenever we want to allocate a slab for a memcg, we access this array to get per-memcg cache to allocate from (see memcg_kmem_get_cache()). The access must be lock-free for performance reasons, so we should use barriers to assert the kmem_cache is up-to-date. First, we should place a write barrier immediately before setting the pointer to it in the memcg_caches array in order to make sure nobody will see a partially initialized object. Second, we should issue a read barrier before dereferencing the pointer to conform to the write barrier. However, currently the barrier usage looks rather strange. We have a write barrier *after* setting the pointer and a read barrier *before* reading the pointer, which is incorrect. This patch fixes this. Signed-off-by: Vladimir Davydov <vdavydov@parallels.com> Cc: Michal Hocko <mhocko@suse.cz> Cc: Glauber Costa <glommer@gmail.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Balbir Singh <bsingharora@gmail.com> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: Christoph Lameter <cl@linux.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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1aa1325425
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959c8963fc
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@ -3274,12 +3274,14 @@ void memcg_register_cache(struct kmem_cache *s)
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list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
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mutex_unlock(&memcg->slab_caches_mutex);
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root->memcg_params->memcg_caches[id] = s;
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/*
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* the readers won't lock, make sure everybody sees the updated value,
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* so they won't put stuff in the queue again for no reason
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* Since readers won't lock (see cache_from_memcg_idx()), we need a
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* barrier here to ensure nobody will see the kmem_cache partially
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* initialized.
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*/
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wmb();
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smp_wmb();
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root->memcg_params->memcg_caches[id] = s;
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}
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void memcg_unregister_cache(struct kmem_cache *s)
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@ -3605,7 +3607,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
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gfp_t gfp)
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{
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struct mem_cgroup *memcg;
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int idx;
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struct kmem_cache *memcg_cachep;
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VM_BUG_ON(!cachep->memcg_params);
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VM_BUG_ON(!cachep->memcg_params->is_root_cache);
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@ -3619,15 +3621,9 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
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if (!memcg_can_account_kmem(memcg))
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goto out;
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idx = memcg_cache_id(memcg);
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/*
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* barrier to mare sure we're always seeing the up to date value. The
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* code updating memcg_caches will issue a write barrier to match this.
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*/
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read_barrier_depends();
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if (likely(cache_from_memcg_idx(cachep, idx))) {
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cachep = cache_from_memcg_idx(cachep, idx);
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memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
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if (likely(memcg_cachep)) {
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cachep = memcg_cachep;
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goto out;
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}
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12
mm/slab.h
12
mm/slab.h
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@ -163,9 +163,19 @@ static inline const char *cache_name(struct kmem_cache *s)
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static inline struct kmem_cache *
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cache_from_memcg_idx(struct kmem_cache *s, int idx)
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{
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struct kmem_cache *cachep;
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if (!s->memcg_params)
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return NULL;
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return s->memcg_params->memcg_caches[idx];
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cachep = s->memcg_params->memcg_caches[idx];
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/*
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* Make sure we will access the up-to-date value. The code updating
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* memcg_caches issues a write barrier to match this (see
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* memcg_register_cache()).
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*/
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smp_read_barrier_depends();
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return cachep;
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}
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static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
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