linux-kernel - Re: [PATCH v5 5/7] mm: rework non-root kmem

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Date:   Tue, 28 May 2019 18:03:53 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        cgroups@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v5 5/7] mm: rework non-root kmem_cache lifecycle
 management

On Tue, May 21, 2019 at 01:07:33PM -0700, Roman Gushchin wrote:
> +	arr = rcu_dereference(cachep->memcg_params.memcg_caches);
> +
> +	/*
> +	 * Make sure we will access the up-to-date value. The code updating
> +	 * memcg_caches issues a write barrier to match this (see
> +	 * memcg_create_kmem_cache()).
> +	 */
> +	memcg_cachep = READ_ONCE(arr->entries[kmemcg_id]);

READ_ONCE() isn't an SMP barrier, it just prevents compiler
muckery. This needs an explicit smp_rmb() to pair with the smp_wmb()
on the other side.

I realize you're only moving this code, but it would be good to fix
that up while you're there.