On Thu, Dec 19, 2013 at 11:07 AM, Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>