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. It hides all the juicy details about memory barriers. Besides that nothing prevents us from freeing from rcu callback. Or? > > Btw. cache_from_memcg_idx is desperately asking for a comment about > > required locking. > > Actually, I placed a reference to the comment there ;-) but no problem, > I move it to cache_from_memcg_idx(). > > Thanks. -- Michal Hocko SUSE Labs _______________________________________________ Devel mailing list Devel@openvz.org https://lists.openvz.org/mailman/listinfo/devel
