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

[Zqiang <qiang.zhang@linux.dev>]

The smp_mb() after kthread_run() in rcu_spawn_tasks_kthread_generic()
from early 'commit eacd6f04a133 ("rcu-tasks: Move Tasks RCU to its
own file")' and 'commit 84a8f446ffd7 ("rcu: Defer rcu_tasks_kthread()
creation till first call_rcu_tasks()")', the 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

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

The rq->lock/unlock from wake_up_process() in kthread_run() and
__schedule() provides a memory barrier when the kthread is first
scheduled, this ensures the kthread itself 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>
---
 kernel/rcu/tasks.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 5a8db89135eb..18ab335665e3 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -395,7 +395,6 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
 		raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags);
 	}
 	rcu_read_unlock();
-	/* We can't create the thread unless interrupts are enabled. */
 	if (needwake && READ_ONCE(rtp->kthread_ptr))
 		irq_work_queue(&rtpcp->rtp_irq_work);
 }
@@ -681,7 +680,6 @@ 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
-- 
2.17.1