rcu: Fix missing nocb gp wake on rcu_barrier()
Upon entraining a callback to a NOCB CPU, no further wake up is
issued on the corresponding nocb_gp kthread. As a result, the callback
and all the subsequent ones on that CPU may be ignored, at least until
an RCU_NOCB_WAKE_FORCE timer is ever armed or another NOCB CPU belonging
to the same group enqueues a callback on an empty queue.
Here is a possible bad scenario:
1) CPU 0 is NOCB unlike all other CPUs.
2) CPU 0 queues a callback
2) The grace period related to that callback elapses
3) The callback is moved to the done list (but is not invoked yet),
there are no more pending callbacks for CPU 0
4) CPU 1 calls rcu_barrier() and sends an IPI to CPU 0
5) CPU 0 entrains the callback but doesn't wake up nocb_gp
6) CPU 1 blocks forever, unless CPU 0 ever queues enough further
callbacks to arm an RCU_NOCB_WAKE_FORCE timer.
Make sure the necessary wake up is produced whenever necessary.
Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
3 files changed
