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cgroup: fix umount vs cgroup_event_remove() race 

commit 5db9a4d99b
 Author: Tejun Heo <tj@kernel.org>
 Date:   Sat Jul 7 16:08:18 2012 -0700

     cgroup: fix cgroup hierarchy umount race

This commit fixed a race caused by the dput() in css_dput_fn(), but
the dput() in cgroup_event_remove() can also lead to the same BUG().

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
This commit is contained in:
Li Zefan 2013-06-18 18:41:10 +08:00 committed by Tejun Heo
parent 084457f284
commit 1c8158eeae
1 changed files with 19 additions and 6 deletions
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25
kernel/cgroup.c
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@ -3821,6 +3821,23 @@ static int cgroup_write_notify_on_release(struct cgroup *cgrp,
return 0;
}
/*
* When dput() is called asynchronously, if umount has been done and
* then deactivate_super() in cgroup_free_fn() kills the superblock,
* there's a small window that vfs will see the root dentry with non-zero
* refcnt and trigger BUG().
*
* That's why we hold a reference before dput() and drop it right after.
*/
static void cgroup_dput(struct cgroup *cgrp)
{
struct super_block *sb = cgrp->root->sb;
atomic_inc(&sb->s_active);
dput(cgrp->dentry);
deactivate_super(sb);
}
/*
* Unregister event and free resources.
*
@ -3841,7 +3858,7 @@ static void cgroup_event_remove(struct work_struct *work)
eventfd_ctx_put(event->eventfd);
kfree(event);
dput(cgrp->dentry);
cgroup_dput(cgrp);
}
/*
@ -4129,12 +4146,8 @@ static void css_dput_fn(struct work_struct *work)
{
struct cgroup_subsys_state *css =
container_of(work, struct cgroup_subsys_state, dput_work);
struct dentry *dentry = css->cgroup->dentry;
struct super_block *sb = dentry->d_sb;
atomic_inc(&sb->s_active);
dput(dentry);
deactivate_super(sb);
cgroup_dput(css->cgroup);
}
static void css_release(struct percpu_ref *ref)