linux-kernel - Re: [PATCH 2/9] slab: remove synchronous rcu_barrier() call in memcg cache release path

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Date:   Sat, 14 Jan 2017 10:19:21 -0500
From:   Tejun Heo <tj@...nel.org>
To:     Vladimir Davydov <vdavydov@...antool.org>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....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, Vladimir.

On Sat, Jan 14, 2017 at 04:19:39PM +0300, Vladimir Davydov wrote:
> On Sat, Jan 14, 2017 at 12:54:42AM -0500, Tejun Heo wrote:
> > This patch updates the cache release path so that it simply uses
> > call_rcu() instead of the synchronous rcu_barrier() + custom batching.
> > This doesn't cost more while being logically simpler and way more
> > scalable.
> 
> The point of rcu_barrier() is to wait until all rcu calls freeing slabs
> from the cache being destroyed are over (rcu_free_slab, kmem_rcu_free).
> I'm not sure if call_rcu() guarantees that for all rcu implementations
> too. If it did, why would we need rcu_barrier() at all?

Yeah, I had a similar question and scanned its users briefly.  Looks
like it's used in combination with ctors so that its users can
opportunistically dereference objects and e.g. check ids / state /
whatever without worrying about the objects' lifetimes.

Thanks.

-- 
tejun