On 12/19/2013 01:36 PM, Michal Hocko wrote: > On Thu 19-12-13 13:29:59, Vladimir Davydov wrote: >> On 12/19/2013 01:21 PM, Michal Hocko wrote: >>> On Thu 19-12-13 13:16:01, Vladimir Davydov wrote: >>>> On 12/19/2013 01:10 PM, Michal Hocko wrote: >>>>> 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. >>>> Because it's not RCU, IMO. RCU implies freeing the old version after a >>>> grace period, while kmem_caches are freed immediately. We simply want to >>>> be sure the kmem_cache is fully initialized. And we do not require >>>> calling this in an RCU critical section. >>> And you can use rcu_dereference and rcu_assign for that as well. >> rcu_dereference() will complain if called outside an RCU critical >> section, while cache_from_memcg_idx() is called w/o RCU protection from >> some places. > Does anything prevents us from using RCU from those callers as well? Yes, take a look at kmem_cache_destroy_memcg_children(), for instance. We call cancel_work_sync() there on a cache obtained via cache_from_memcg_idx(). Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>