lkml.org 
[lkml]   [2019]   [Jun]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v6 01/10] mm: add missing smp read barrier on getting memcg kmem_cache pointer
On Sun, Jun 09, 2019 at 03:10:52PM +0300, Vladimir Davydov wrote:
> On Tue, Jun 04, 2019 at 07:44:45PM -0700, Roman Gushchin wrote:
> > Johannes noticed that reading the memcg kmem_cache pointer in
> > cache_from_memcg_idx() is performed using READ_ONCE() macro,
> > which doesn't implement a SMP barrier, which is required
> > by the logic.
> >
> > Add a proper smp_rmb() to be paired with smp_wmb() in
> > memcg_create_kmem_cache().
> >
> > The same applies to memcg_create_kmem_cache() itself,
> > which reads the same value without barriers and READ_ONCE().
> >
> > Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > ---
> > mm/slab.h | 1 +
> > mm/slab_common.c | 3 ++-
> > 2 files changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 739099af6cbb..1176b61bb8fc 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -260,6 +260,7 @@ cache_from_memcg_idx(struct kmem_cache *s, int idx)
> > * memcg_caches issues a write barrier to match this (see
> > * memcg_create_kmem_cache()).
> > */
> > + smp_rmb();
> > cachep = READ_ONCE(arr->entries[idx]);
>
> Hmm, we used to have lockless_dereference() here, but it was replaced
> with READ_ONCE some time ago. The commit message claims that READ_ONCE
> has an implicit read barrier in it.

Thanks for catching this Vladimir. I wasn't aware of this change to
the memory model. Indeed, we don't need to change anything here.

\
 
 \ /
  Last update: 2019-06-10 22:34    [W:0.077 / U:20.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site