Subject: [merged] memcg-slab-fix-barrier-usage-when-accessing-memcg_caches.patch removed from -mm tree To: vdavydov@xxxxxxxxxxxxx,bsingharora@xxxxxxxxx,cl@xxxxxxxxx,glommer@xxxxxxxxx,hannes@xxxxxxxxxxx,kamezawa.hiroyu@xxxxxxxxxxxxxx,mhocko@xxxxxxx,penberg@xxxxxxxxxx,mm-commits@xxxxxxxxxxxxxxx From: akpm@xxxxxxxxxxxxxxxxxxxx Date: Fri, 24 Jan 2014 10:58:41 -0800 The patch titled Subject: memcg, slab: fix barrier usage when accessing memcg_caches has been removed from the -mm tree. Its filename was memcg-slab-fix-barrier-usage-when-accessing-memcg_caches.patch This patch was dropped because it was merged into mainline or a subsystem tree ------------------------------------------------------ From: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> Subject: 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@xxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxx> Cc: Glauber Costa <glommer@xxxxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Balbir Singh <bsingharora@xxxxxxxxx> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memcontrol.c | 24 ++++++++++-------------- mm/slab.h | 12 +++++++++++- 2 files changed, 21 insertions(+), 15 deletions(-) diff -puN mm/memcontrol.c~memcg-slab-fix-barrier-usage-when-accessing-memcg_caches mm/memcontrol.c --- a/mm/memcontrol.c~memcg-slab-fix-barrier-usage-when-accessing-memcg_caches +++ a/mm/memcontrol.c @@ -3274,12 +3274,14 @@ void memcg_register_cache(struct kmem_ca list_add(&s->memcg_params->list, &memcg->memcg_slab_caches); mutex_unlock(&memcg->slab_caches_mutex); - root->memcg_params->memcg_caches[id] = s; /* - * 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(); + + root->memcg_params->memcg_caches[id] = s; } void memcg_unregister_cache(struct kmem_cache *s) @@ -3605,7 +3607,7 @@ struct kmem_cache *__memcg_kmem_get_cach 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); @@ -3619,15 +3621,9 @@ struct kmem_cache *__memcg_kmem_get_cach 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 -puN mm/slab.h~memcg-slab-fix-barrier-usage-when-accessing-memcg_caches mm/slab.h --- a/mm/slab.h~memcg-slab-fix-barrier-usage-when-accessing-memcg_caches +++ a/mm/slab.h @@ -163,9 +163,19 @@ static inline const char *cache_name(str 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]; + + /* + * Make sure we will access the up-to-date value. The code updating + * memcg_caches issues a write barrier to match this (see + * memcg_register_cache()). + */ + smp_read_barrier_depends(); + return cachep; } static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s) _ Patches currently in -mm which might be from vdavydov@xxxxxxxxxxxxx are origin.patch mm-vmscan-respect-numa-policy-mask-when-shrinking-slab-on-direct-reclaim.patch mm-vmscan-move-call-to-shrink_slab-to-shrink_zones.patch mm-vmscan-remove-shrink_control-arg-from-do_try_to_free_pages.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html