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Date: Tue, 17 Jan 2017 08:37:45 -0800 From: Tejun Heo <tj@...nel.org> To: Joonsoo Kim <iamjoonsoo.kim@....com> Cc: Vladimir Davydov <vdavydov@...antool.org>, cl@...ux.com, penberg@...nel.org, rientjes@...gle.com, akpm@...ux-foundation.org, jsvana@...com, hannes@...xchg.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org, kernel-team@...com Subject: Re: [PATCH 2/9] slab: remove synchronous rcu_barrier() call in memcg cache release path Hello, Joonsoo. On Tue, Jan 17, 2017 at 09:07:54AM +0900, Joonsoo Kim wrote: > Long time no see! :) Yeah, happy new year! > IIUC, rcu_barrier() here prevents to destruct the kmem_cache until all > slab pages in it are freed. These slab pages are freed through call_rcu(). Hmm... why do we need that tho? SLAB_DESTROY_BY_RCU only needs to protect the slab pages, not kmem cache struct. I thought that this was because kmem cache destruction is allowed to release pages w/o RCU delaying it. > Your patch changes it to another call_rcu() and, I think, if sequence of > executing rcu callbacks is the same with sequence of adding rcu > callbacks, it would work. However, I'm not sure that it is > guaranteed by RCU API. Am I missing something? The call sequence doesn't matter. Whether you're using call_rcu() or rcu_barrier(), you're just waiting for a grace period to pass before continuing. It doens't give any other ordering guarantees, so the new code should be equivalent to the old one except for being asynchronous. Thanks. -- tejun
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