Subject: + memcg-slab-fix-barrier-usage-when-accessing-memcg_caches.patch added to -mm tree
To: vdavydov@xxxxxxxxxxxxx,bsingharora@xxxxxxxxx,cl@xxxxxxxxx,glommer@xxxxxxxxx,hannes@xxxxxxxxxxx,kamezawa.hiroyu@xxxxxxxxxxxxxx,mhocko@xxxxxxx,penberg@xxxxxxxxxx
From: akpm@xxxxxxxxxxxxxxxxxxxx
Date: Mon, 06 Jan 2014 14:56:21 -0800
The patch titled
Subject: memcg, slab: fix barrier usage when accessing memcg_caches
has been added to the -mm tree. Its filename is
memcg-slab-fix-barrier-usage-when-accessing-memcg_caches.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/memcg-slab-fix-barrier-usage-when-accessing-memcg_caches.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/memcg-slab-fix-barrier-usage-when-accessing-memcg_caches.patch
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------------------------------------------------------
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
@@ -3234,12 +3234,14 @@ 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)
@@ -3565,7 +3567,7 @@ 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);
@@ -3579,15 +3581,9 @@ 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
memcg-fix-kmem_account_flags-check-in-memcg_can_account_kmem.patch
memcg-make-memcg_update_cache_sizes-static.patch
memcg-do-not-use-vmalloc-for-mem_cgroup-allocations.patch
slab-clean-up-kmem_cache_create_memcg-error-handling.patch
memcg-slab-kmem_cache_create_memcg-fix-memleak-on-fail-path.patch
memcg-slab-clean-up-memcg-cache-initialization-destruction.patch
memcg-slab-fix-barrier-usage-when-accessing-memcg_caches.patch
memcg-fix-possible-null-deref-while-traversing-memcg_slab_caches-list.patch
memcg-slab-fix-races-in-per-memcg-cache-creation-destruction.patch
memcg-get-rid-of-kmem_cache_dup.patch
slab-do-not-panic-if-we-fail-to-create-memcg-cache.patch
memcg-slab-rcu-protect-memcg_params-for-root-caches.patch
memcg-remove-kmem_accounted_activated-flag.patch
memcg-rework-memcg_update_kmem_limit-synchronization.patch
linux-next.patch
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