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

[Joonsoo Kim]

On Thu, Mar 31, 2016 at 01:53:14PM +0300, Nikolay Borisov wrote:
> 
> 
> On 03/28/2016 08:26 AM, js1304@xxxxxxxxx wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> > 
> > 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.