cgroup: fix offlining child waiting in cgroup_subtree_control_write()
cgroup_subtree_control_write() waits for offline to complete child-by-child before enabling a controller; however, it has a couple bugs. * It doesn't initialize the wait_queue_t. This can lead to infinite hang on the following schedule() among other things. * It forgets to pin the child before releasing cgroup_tree_mutex and performing schedule(). The child may already be gone by the time it wakes up and invokes finish_wait(). Pin the child being waited on. Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Li Zefan <lizefan@huawei.com>
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@ -2594,16 +2594,18 @@ retry:
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* cases, wait till it's gone using offline_waitq.
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*/
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cgroup_for_each_live_child(child, cgrp) {
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wait_queue_t wait;
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DEFINE_WAIT(wait);
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if (!cgroup_css(child, ss))
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continue;
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cgroup_get(child);
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prepare_to_wait(&child->offline_waitq, &wait,
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TASK_UNINTERRUPTIBLE);
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mutex_unlock(&cgroup_tree_mutex);
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schedule();
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finish_wait(&child->offline_waitq, &wait);
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cgroup_put(child);
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goto retry;
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}
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