Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling __kmem_cache_shrink()

From	Joonsoo Kim <iamjoonsoo.kim@lge.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling __kmem_cache_shrink()
Date	2016-04-01 04:20 +0200
Message-ID	<riWQO-62F-5@gated-at.bofh.it> (permalink)
References	<rhxUt-2rs-3@gated-at.bofh.it> <rhxUt-2rs-1@gated-at.bofh.it> <riIuu-3Hf-1@gated-at.bofh.it>
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On Thu, Mar 31, 2016 at 01:53:14PM +0300, Nikolay Borisov wrote:
> 
> 
> 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.

Good idea. I'm not sure if lockdep_assert is best fit but I will add
something to check it rather than just adding the comment.

Thanks.

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[PATCH 01/11] mm/slab: hold a slab_mutex when calling __kmem_cache_shrink() js1304@gmail.com - 2016-03-28 07:30 +0200
  Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling  __kmem_cache_shrink() Christoph Lameter <cl@linux.com> - 2016-03-29 03:00 +0200
    Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling  __kmem_cache_shrink() Joonsoo Kim <iamjoonsoo.kim@lge.com> - 2016-03-30 10:20 +0200
  Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling  __kmem_cache_shrink() Nikolay Borisov <kernel@kyup.com> - 2016-03-31 13:00 +0200
    Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling  __kmem_cache_shrink() Joonsoo Kim <iamjoonsoo.kim@lge.com> - 2016-04-01 04:20 +0200

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