[PATCH 09/10] rcu-tasks: Remove smp_mb() in rcu_spawn_tasks_kthread_generic()

["Paul E. McKenney" <paulmck@kernel.org>]

From: Zqiang <qiang.zhang@linux.dev>

For the smp_mb() after kthread_run() in rcu_spawn_tasks_kthread_generic()
from these commits:

'commit eacd6f04a133 ("rcu-tasks: Move Tasks RCU to its own file")'
'commit 84a8f446ffd7 ("rcu: Defer rcu_tasks_kthread() creation till
 first call_rcu_tasks()")'

the memory order pairing as follows:

rcu_spawn_tasks_kthread()
->t = kthread_run(rcu_tasks_kthread, ...);
->smp_mb(); /* Ensure others see full kthread. */
->WRITE_ONCE(rcu_tasks_kthread_ptr, t);

							call_rcu_tasks()
					      		->if (READ_ONCE(rcu_tasks_kthread_ptr))
					         	  ->wake_up(&rcu_tasks_cbs_wq)
							    ->try_to_wake_up()
							      lock pi_lock
							      ->smp_mb__after_spinlock()
								//see full kthread

Because the 'commit d119357d0743 ("rcu-tasks: Treat only synchronous
grace periods urgently")' moved the kthread_ptr assignment into the
rcu_tasks_kthread() function, the following memory order pairings are
sufficient:

The runq's raw_spinlock/unlock(or smp_mb__after_spinlock()) from
wake_up_process() in kthread_run() and __schedule() provides memory order
barrier when the kthread is first scheduled, this ensures the kthread's
func observes all of the kthread's initialization.

The kthread's smp_store_release(&rtp->kthread_ptr, ...) in
rcu_tasks_kthread() and smp_load_acquire(&rtp->kthread_ptr)
in call_rcu_tasks_generic() compose release/acquire pairing, the
cumulativity of smp_store_release() propagates visibility of the kthread's
initialization through the scheduler chain.

This commit therefore remove smp_mb() in rcu_spawn_tasks_kthread_generic().

Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tasks.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index d9e1e53f4ccf31..79aa5d51388c45 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -681,7 +681,6 @@ static void __init rcu_spawn_tasks_kthread_generic(struct rcu_tasks *rtp)
 	t = kthread_run(rcu_tasks_kthread, rtp, "%s_kthread", rtp->kname);
 	if (WARN_ONCE(IS_ERR(t), "%s: Could not start %s grace-period kthread, OOM is now expected behavior\n", __func__, rtp->name))
 		return;
-	smp_mb(); /* Ensure others see full kthread. */
 }
 
 #ifndef CONFIG_TINY_RCU
-- 
2.40.1