[PATCH] rcu-tasks: Fix possible boot-time tests failed for the call_rcu_tasks()

[PATCH] rcu-tasks: Fix possible boot-time tests failed for the call_rcu_tasks()

[Zqiang]

The following scenarios will cause the call_rcu_tasks() boot-time
tests failed:

                     CPU0								CPU1

rcu_init_tasks_generic()
->rcu_tasks_initiate_self_tests()
  ->call_rcu_tasks_trace(&tests[1].rh, test_rcu_tasks_callback)
    ->call_rcu_tasks_generic()
      ->havekthread = smp_load_acquire(&rtp->kthread_ptr)
       "The havekthread is false"
    ....
								       rcu_tasks_kthread()
								       ->smp_store_release(&rtp->kthread_ptr, current)
								         ->rcu_tasks_one_gp()
								           ->rcuwait_wait_event()
								             ->rcu_tasks_need_gpcb()
								               ->for (cpu = 0; cpu < dequeue_limit; cpu++)
								                 ->rcu_segcblist_n_cbs(&rtpcp->cblist) == 0
								           ->schedule()

     ->raw_spin_trylock_rcu_node()
     ->needwake = (func == wakeme_after_rcu) ||
     (rcu_segcblist_n_cbs(&rtpcp->cblist) == rcu_task_lazy_lim)

       "the rcu_task_lazy_lim default value is 32, and the
        func pointer is test_rcu_tasks_callback, lead to needwake
        is false."

     ->if (havekthread && !needwake && !timer_pending(&rtpcp->lazy_timer))
        "the havekthread is false, will not enter here."
     ....

        "the needwake is false lead to rtp_irq_work can not queue,
         even if the rtp->kthread_ptr already exists at this point."

      ->if (needwake && READ_ONCE(rtp->kthread_ptr))
        ->irq_work_queue(&rtpcp->rtp_irq_work)

For the above scenarios, if the call_rcu_tasks() is not called again
afterward, the rcu_tasks_kthread will not have a chance to be wakeup,
the test_rcu_tasks_callback() will never be called, the boot-time tests
failed can happen, this commit therefore check havekthread variable, if
it's false and the rtpcp->cblist is empty, set needwake variable is true,
if the rtp->kthread_ptr exist, the rtpcp->rtp_irq_work can be queued to
wakeup rcu_tasks_kthread.

Fixes: d119357d0743 ("rcu-tasks: Treat only synchronous grace periods urgently")
Signed-off-by: Zqiang <qiang.zhang@linux.dev>
---
 
 Possible scenarios for reproduction: https://syzkaller.appspot.com/bug?extid=251e9abcdac140e7ec74

 kernel/rcu/tasks.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 48f0d803c8e2..f4da5fad70f5 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -373,7 +373,8 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
 	// Queuing callbacks before initialization not yet supported.
 	if (WARN_ON_ONCE(!rcu_segcblist_is_enabled(&rtpcp->cblist)))
 		rcu_segcblist_init(&rtpcp->cblist);
-	needwake = (func == wakeme_after_rcu) ||
+	needwake = (!havekthread && rcu_segcblist_empty(&rtpcp->cblist)) ||
+		   (func == wakeme_after_rcu) ||
 		   (rcu_segcblist_n_cbs(&rtpcp->cblist) == rcu_task_lazy_lim);
 	if (havekthread && !needwake && !timer_pending(&rtpcp->lazy_timer)) {
 		if (rtp->lazy_jiffies)
-- 
2.17.1