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. > 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. -- 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>