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

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Date:	Wed, 30 Mar 2016 17:11:16 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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
Subject: Re: [PATCH 01/11] mm/slab: hold a slab_mutex when calling
 __kmem_cache_shrink()

On Mon, Mar 28, 2016 at 07:50:36PM -0500, Christoph Lameter wrote:
> On Mon, 28 Mar 2016, js1304@...il.com wrote:
> 
> > 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.
> 
> Ummm.. The mutex taking is added to common code. So this will also affect
> SLUB.  The patch needs to consider this. Do we want to force all
> allocators to run shrinking only when holding the lock? SLUB does not
> need to hold the mutex. And frankly the mutex is for reconfiguration of
> metadata which is *not* occurring here. A shrink operation does not do
> that. Can we figure out a slab specific way of handling synchronization
> in the strange free/realloc cycle?
> 
> It seems that taking the node lock is the appropriate level of
> synchrnonization since the concern is with the contents of a shared cache
> at that level. There is no change of metadata which would require the
> mutex.

Okay. I will fix it.

Thanks.