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SubjectRe: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy.
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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