[Devel] [PATCH 4/6] memcg, slab: check and init memcg_cahes under slab_mutex

[Glauber Costa]

On Thu, Dec 19, 2013 at 11:07 AM, Vladimir Davydov
<vdavydov at parallels.com> wrote:
> On 12/18/2013 09:41 PM, Michal Hocko wrote:
>> On Wed 18-12-13 17:16:55, Vladimir Davydov wrote:
>>> The memcg_params::memcg_caches array can be updated concurrently from
>>> memcg_update_cache_size() and memcg_create_kmem_cache(). Although both
>>> of these functions take the slab_mutex during their operation, the
>>> latter checks if memcg's cache has already been allocated w/o taking the
>>> mutex. This can result in a race as described below.
>>>
>>> Asume two threads schedule kmem_cache creation works for the same
>>> kmem_cache of the same memcg from __memcg_kmem_get_cache(). One of the
>>> works successfully creates it. Another work should fail then, but if it
>>> interleaves with memcg_update_cache_size() as follows, it does not:
>> I am not sure I understand the race. memcg_update_cache_size is called
>> when we start accounting a new memcg or a child is created and it
>> inherits accounting from the parent. memcg_create_kmem_cache is called
>> when a new cache is first allocated from, right?
>
> memcg_update_cache_size() is called when kmem accounting is activated
> for a memcg, no matter how.
>
> memcg_create_kmem_cache() is scheduled from __memcg_kmem_get_cache().
> It's OK to have a bunch of such methods trying to create the same memcg
> cache concurrently, but only one of them should succeed.
>
>> Why cannot we simply take slab_mutex inside memcg_create_kmem_cache?
>> it is running from the workqueue context so it should clash with other
>> locks.
>
> Hmm, Glauber's code never takes the slab_mutex inside memcontrol.c. I
> have always been wondering why, because it could simplify flow paths
> significantly (e.g. update_cache_sizes() -> update_all_caches() ->
> update_cache_size() - from memcontrol.c to slab_common.c and back again
> just to take the mutex).
>

Because that is a layering violation and exposes implementation
details of the slab to
the outside world. I agree this would make things a lot simpler, but
please check with Christoph
if this is acceptable before going forward.