Subject: Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling
 __kmem_cache_shrink()
To: js1304@gmail.com, Andrew Morton <akpm@linux-foundation.org>
References: <1459142821-20303-1-git-send-email-iamjoonsoo.kim@lge.com>
 <1459142821-20303-2-git-send-email-iamjoonsoo.kim@lge.com>
Cc: Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Joonsoo Kim <iamjoonsoo.kim@lge.com>
From: Nikolay Borisov <kernel@kyup.com>
Message-ID: <56FD019A.10906@kyup.com>
Date: Thu, 31 Mar 2016 13:53:14 +0300
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On 03/28/2016 08:26 AM, js1304@gmail.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> 
> Major kmem_cache metadata in slab subsystem is synchronized with
> the slab_mutex. In SLAB, if some of them is changed, node's shared
> array cache would be freed and re-populated. If __kmem_cache_shrink()
> is called at the same time, it will call drain_array() with n->shared
> without holding node lock so problem can happen.
> 
> We can fix this small theoretical race condition by holding node lock
> in drain_array(), but, holding a slab_mutex in kmem_cache_shrink()
> looks more appropriate solution because stable state would make things
> less error-prone and this is not performance critical path.
> 
> In addtion, annotate on SLAB functions.

Just a nit but would it not be better instead of doing comment-style
annotation to use lockdep_assert_held/_once. In both cases for someone
to understand what locks have to be held will go and read the source. In
my mind it's easier to miss a comment line, rather than the
lockdep_assert. Furthermore in case lockdep is enabled a locking
violation would spew useful info to dmesg.