Messages in this thread | | | Date | Wed, 12 Nov 2008 17:15:49 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy. |
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On Wed, 12 Nov 2008 14:58:06 +0800 Li Zefan <lizf@cn.fujitsu.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. > > > > 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.
Thank you for review.
-Kame
> > > 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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