[tip: core/rcu] rcu: Invoke rcu_spawn_core_kthreads() from rcu_spawn_gp_kthread()

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     8e4b1d2bc198e34b48fc7cc3a3c5a2fcb269e271
Gitweb:        https://git.kernel.org/tip/8e4b1d2bc198e34b48fc7cc3a3c5a2fcb269e271
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Wed, 31 Mar 2021 10:59:05 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 10 May 2021 16:22:54 -07:00

rcu: Invoke rcu_spawn_core_kthreads() from rcu_spawn_gp_kthread()

Currently, rcu_spawn_core_kthreads() is invoked via an early_initcall(),
which works, except that rcu_spawn_gp_kthread() is also invoked via an
early_initcall() and rcu_spawn_core_kthreads() relies on adjustments to
kthread_prio that are carried out by rcu_spawn_gp_kthread().  There is
no guaranttee of ordering among early_initcall() handlers, and thus no
guarantee that kthread_prio will be properly checked and range-limited
at the time that rcu_spawn_core_kthreads() needs it.

In most cases, this bug is harmless.  After all, the only reason that
rcu_spawn_gp_kthread() adjusts the value of kthread_prio is if the user
specified a nonsensical value for this boot parameter, which experience
indicates is rare.

Nevertheless, a bug is a bug.  This commit therefore causes the
rcu_spawn_core_kthreads() function to be invoked directly from
rcu_spawn_gp_kthread() after any needed adjustments to kthread_prio have
been carried out.

Fixes: 48d07c04b4cc ("rcu: Enable elimination of Tree-RCU softirq processing")
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 06f3de9..2532e58 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2889,7 +2889,6 @@ static int __init rcu_spawn_core_kthreads(void)
 		  "%s: Could not start rcuc kthread, OOM is now expected behavior\n", __func__);
 	return 0;
 }
-early_initcall(rcu_spawn_core_kthreads);
 
 /*
  * Handle any core-RCU processing required by a call_rcu() invocation.
@@ -4450,6 +4449,7 @@ static int __init rcu_spawn_gp_kthread(void)
 	wake_up_process(t);
 	rcu_spawn_nocb_kthreads();
 	rcu_spawn_boost_kthreads();
+	rcu_spawn_core_kthreads();
 	return 0;
 }
 early_initcall(rcu_spawn_gp_kthread);