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synchronize_rcu can be very expensive, averaging 100 ms in some cases. In cgroup_attach_task, it is used to prevent a task->cgroups pointer dereferenced in an RCU read side critical section from being invalidated, by delaying the call to put_css_set until after an RCU grace period. To avoid the call to synchronize_rcu, make the put_css_set call rcu-safe by moving the deletion of the css_set links into free_css_set_work, scheduled by the rcu callback free_css_set_rcu. The decrement of the cgroup refcount is no longer synchronous with the call to put_css_set, which can result in the cgroup refcount staying positive after the last call to cgroup_attach_task returns. To allow the cgroup to be deleted with cgroup_rmdir synchronously after cgroup_attach_task, have rmdir check the refcount of all associated css_sets. If cgroup_rmdir is called on a cgroup for which the css_sets all have refcount zero but the cgroup refcount is nonzero, reuse the rmdir waitqueue to block the rmdir until free_css_set_work is called. Signed-off-by: Colin Cross <ccross@android.com>