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 <[email protected]>
---
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
