Messages in this thread | | | Date | Wed, 12 Nov 2008 13:55:48 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy. |
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On Wed, 12 Nov 2008 10:23:55 +0530 Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> KAMEZAWA Hiroyuki wrote: > > Balbir, Paul, Li, How about this ? > > = > > As Balbir pointed out, memcg's pre_destroy handler has potential deadlock. > > > > It has following lock sequence. > > > > cgroup_mutex (cgroup_rmdir) > > -> pre_destroy > > -> mem_cgroup_pre_destroy > > -> force_empty > > -> lru_add_drain_all-> > > -> schedule_work_on_all_cpus > > -> get_online_cpus -> cpuhotplug.lock. > > > > But, cpuset has following. > > cpu_hotplug.lock (call notifier) > > -> cgroup_mutex. (within notifier) > > > > Then, this lock sequence should be fixed. > > > > Considering how pre_destroy works, it's not necessary to holding > > cgroup_mutex() while calling it. > > > > As side effect, we don't have to wait at this mutex while memcg's force_empty > > works.(it can be long when there are tons of pages.) > > > > Note: memcg is an only user of pre_destroy, now. > > > > I thought about this and it seems promising. My concern is that with > cgroup_mutex given, the state of cgroup within pre-destroy will be > unpredictable. I suspect, if pre-destory really needs cgroup_mutex, we can hold > it within pre-destroy. > I agree.
> BTW, your last check, does not seem right > > + if (atomic_read(&cgrp->count) > + || list_empty(&cgrp->children) > > Why should list_empty() result in EBUSY, shouldn't it be !list_empty()? > > + || cgroup_has_css_refs(cgrp)) { > Oh, my bad...
will fix soon.
Thanks, -Kame
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