On 11/30/2012 01:24 PM, Michal Hocko wrote: > 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? > Apparently, the way Tejun fixed this for cpuset was by using the "attach_in_progress" indicator, that IIUC, is flipped up in ->can_attach, and down in ->attach. A similar scheme would work for us. And we should also be using a similar scheme for cgroup creation: the css is not really connected to the parent until after memcg_alloc_css. So if we use the memcg iterator to figure out if children exist, we may get a race where we believe no children exist, but one appear right after. -- 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>