On Fri 30-11-12 13:00:36, Glauber Costa wrote: > On 11/30/2012 07:21 AM, Kamezawa Hiroyuki wrote: > > (2012/11/29 6:34), Tejun Heo wrote: > >> Hello, guys. > >> > >> Depending on cgroup core locking - cgroup_mutex - is messy and makes > >> cgroup prone to locking dependency problems. The current code already > >> has lock dependency loop - memcg nests get_online_cpus() inside > >> cgroup_mutex. cpuset the other way around. > >> > >> Regardless of the locking details, whatever is protecting cgroup has > >> inherently to be something outer to most other locking constructs. > >> cgroup calls into a lot of major subsystems which in turn have to > >> perform subsystem-specific locking. Trying to nest cgroup > >> synchronization inside other locks isn't something which can work > >> well. > >> > >> cgroup now has enough API to allow subsystems to implement their own > >> locking and cgroup_mutex is scheduled to be made private to cgroup > >> core. This patchset makes cpuset implement its own locking instead of > >> relying on cgroup_mutex. > >> > >> cpuset is rather nasty in this respect. Some of it seems to have come > >> from the implementation history - cgroup core grew out of cpuset - but > >> big part stems from cpuset's need to migrate tasks to an ancestor > >> cgroup when an hotunplug event makes a cpuset empty (w/o any cpu or > >> memory). > >> > >> This patchset decouples cpuset locking from cgroup_mutex. After the > >> patchset, cpuset uses cpuset-specific cpuset_mutex instead of > >> cgroup_mutex. This also removes the lockdep warning triggered during > >> cpu offlining (see 0009). > >> > >> Note that this leaves memcg as the only external user of cgroup_mutex. > >> Michal, Kame, can you guys please convert memcg to use its own locking > >> too? > >> > > > > Hmm. let me see....at quick glance cgroup_lock() is used at > > hierarchy policy change > > kmem_limit > > migration policy change > > swapiness change > > oom control > > > > Because all aboves takes care of changes in hierarchy, > > Having a new memcg's mutex in ->create() may be a way. > > > > Ah, hm, Costa is mentioning task-attach. is the task-attach problem in memcg ? > > > > We disallow the kmem limit to be set if a task already exists in the > cgroup. So we can't allow a new task to attach if we are setting the limit. This is racy without additional locking, isn't it? -- Michal Hocko SUSE Labs -- 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>