The smp_mb() after kthread_run() in rcu_spawn_tasks_kthread_generic()
from early commit:
'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
Currently, the 'commit d119357d0743 ("rcu-tasks: Treat only
synchronous grace periods urgently")' moved the kthread_ptr
assignment into the rcu_tasks_kthread(), 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 <[email protected]>
---
kernel/rcu/tasks.h | 1 -
1 file changed, 1 deletion(-)
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 5a8db89135eb..a2ca79d1dd6a 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
--
2.17.1
