linux-kernel - Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling __kmem_cache

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Date:	Thu, 31 Mar 2016 13:53:14 +0300
From:	Nikolay Borisov <kernel@...p.com>
To:	js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling
 __kmem_cache_shrink()



On 03/28/2016 08:26 AM, js1304@...il.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@....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.