Messages in this thread | | | Date | Wed, 12 Nov 2008 14:58:06 +0800 | From | Li Zefan <> | Subject | Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy. |
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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. >
I think it's safe to call cgroup_call_pre_destroy() without cgroup_lock. If cgroup_call_pre_destroy() gets called, it means the cgroup fs has sub-dirs, so any remount/umount will fail, which means root->subsys_list won't be changed during rmdir(), so using for_each_subsys() in cgroup_call_pre_destroy() is safe.
> 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. > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> > > --- > kernel/cgroup.c | 14 +++++++++----- > 1 file changed, 9 insertions(+), 5 deletions(-) > > Index: mmotm-2.6.28-Nov10/kernel/cgroup.c > =================================================================== > --- mmotm-2.6.28-Nov10.orig/kernel/cgroup.c > +++ mmotm-2.6.28-Nov10/kernel/cgroup.c > @@ -2475,10 +2475,7 @@ static int cgroup_rmdir(struct inode *un > mutex_unlock(&cgroup_mutex); > return -EBUSY; > } > - > - parent = cgrp->parent; > - root = cgrp->root; > - sb = root->sb; > + mutex_unlock(&cgroup_mutex); > > /* > * Call pre_destroy handlers of subsys. Notify subsystems > @@ -2486,7 +2483,14 @@ static int cgroup_rmdir(struct inode *un > */ > cgroup_call_pre_destroy(cgrp); > > - if (cgroup_has_css_refs(cgrp)) { > + mutex_lock(&cgroup_mutex); > + parent = cgrp->parent; > + root = cgrp->root; > + sb = root->sb; > + > + if (atomic_read(&cgrp->count) > + || list_empty(&cgrp->children) > + || cgroup_has_css_refs(cgrp)) { > mutex_unlock(&cgroup_mutex); > return -EBUSY; > } >
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