rcu/exp: Remove rcu_par_gp_wq

TREE04 running on short iterations can produce writer stalls of the
following kind:

 ??? Writer stall state RTWS_EXP_SYNC(4) g3968 f0x0 ->state 0x2 cpu 0
 task:rcu_torture_wri state:D stack:14568 pid:83    ppid:2      flags:0x00004000
 Call Trace:
  <TASK>
  __schedule+0x2de/0x850
  ? trace_event_raw_event_rcu_exp_funnel_lock+0x6d/0xb0
  schedule+0x4f/0x90
  synchronize_rcu_expedited+0x430/0x670
  ? __pfx_autoremove_wake_function+0x10/0x10
  ? __pfx_synchronize_rcu_expedited+0x10/0x10
  do_rtws_sync.constprop.0+0xde/0x230
  rcu_torture_writer+0x4b4/0xcd0
  ? __pfx_rcu_torture_writer+0x10/0x10
  kthread+0xc7/0xf0
  ? __pfx_kthread+0x10/0x10
  ret_from_fork+0x2f/0x50
  ? __pfx_kthread+0x10/0x10
  ret_from_fork_asm+0x1b/0x30
  </TASK>

Waiting for an expedited grace period and polling for an expedited
grace period both are operations that internally rely on the same
workqueue performing necessary asynchronous work.

However, a dependency chain is involved between those two operations,
as depicted below:

       ====== CPU 0 =======                          ====== CPU 1 =======

                                                     synchronize_rcu_expedited()
                                                         exp_funnel_lock()
                                                             mutex_lock(&rcu_state.exp_mutex);
    start_poll_synchronize_rcu_expedited
        queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
                                                         synchronize_rcu_expedited_queue_work()
                                                             queue_work(rcu_gp_wq, &rew->rew_work);
                                                         wait_event() // A, wait for &rew->rew_work completion
                                                         mutex_unlock() // B
    //======> switch to kworker

    sync_rcu_do_polled_gp() {
        synchronize_rcu_expedited()
            exp_funnel_lock()
                mutex_lock(&rcu_state.exp_mutex); // C, wait B
                ....
    } // D

Since workqueues are usually implemented on top of several kworkers
handling the queue concurrently, the above situation wouldn't deadlock
most of the time because A then doesn't depend on D. But in case of
memory stress, a single kworker may end up handling alone all the works
in a serialized way. In that case the above layout becomes a problem
because A then waits for D, closing a circular dependency:

	A -> D -> C -> B -> A

This however only happens when CONFIG_RCU_EXP_KTHREAD=n. Indeed
synchronize_rcu_expedited() is otherwise implemented on top of a kthread
worker while polling still relies on rcu_gp_wq workqueue, breaking the
above circular dependency chain.

Fix this with making expedited grace period to always rely on kthread
worker. The workqueue based implementation is essentially a duplicate
anyway now that the per-node initialization is performed by per-node
kthread workers.

Meanwhile the CONFIG_RCU_EXP_KTHREAD switch is still kept around to
manage the scheduler policy of these kthread workers.

Reported-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Joel Fernandes <joel@joelfernandes.org>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Suggested-by: Neeraj upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>