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Date: Thu, 19 Dec 2013 10:10:07 +0100
From: Michal Hocko <mhocko@...e.cz>
To: Vladimir Davydov <vdavydov@...allels.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
cgroups@...r.kernel.org, devel@...nvz.org,
Johannes Weiner <hannes@...xchg.org>,
Glauber Costa <glommer@...il.com>,
Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/6] memcg, slab: cleanup barrier usage when accessing
memcg_caches
On Thu 19-12-13 10:37:27, Vladimir Davydov wrote:
> On 12/18/2013 09:14 PM, Michal Hocko wrote:
> > 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.
>
> In short, we have kmem_cache::memcg_params::memcg_caches is an array of
> pointers to per-memcg caches. We access it lock-free so we should use
> memory barriers during initialization. Obviously we should place a write
> barrier just before we set the pointer in order to make sure nobody will
> see a partially initialized structure. Besides there must be a read
> barrier between reading the pointer and accessing the structure, to
> conform with the write barrier. It's all that similar to rcu_assign and
> rcu_deref. Currently the barrier usage looks rather strange:
>
> memcg_create_kmem_cache:
> initialize kmem
> set the pointer in memcg_caches
> wmb() // ???
>
> __memcg_kmem_get_cache:
> <...>
> read_barrier_depends() // ???
> cachep = root_cache->memcg_params->memcg_caches[memcg_id]
> <...>
Why do we need explicit memory barriers when we can use RCU?
__memcg_kmem_get_cache already dereferences within rcu_read_lock.
Btw. cache_from_memcg_idx is desperately asking for a comment about
required locking.
> Nothing prevents some archs from moving initialization after setting the
> pointer, or reading data before reading the pointer to it.
>
> Of course, I will include a detailed description in the next version of
> this patch.
>
> Thanks.
>
> >> Signed-off-by: Vladimir Davydov <vdavydov@...allels.com>
> >> Cc: Michal Hocko <mhocko@...e.cz>
> >> Cc: Johannes Weiner <hannes@...xchg.org>
> >> Cc: Glauber Costa <glommer@...il.com>
> >> Cc: Christoph Lameter <cl@...ux.com>
> >> Cc: Pekka Enberg <penberg@...nel.org>
> >> Cc: Andrew Morton <akpm@...ux-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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