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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, "Paul E. McKenney" <paulmck@kernel.org>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.4 255/348] rcu: Invoke rcu_spawn_core_kthreads() from rcu_spawn_gp_kthread()
Date:   Mon, 12 Jul 2021 08:10:39 +0200
Message-Id: <20210712060736.660966878@linuxfoundation.org>
In-Reply-To: <20210712060659.886176320@linuxfoundation.org>
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From: Paul E. McKenney <paulmck@kernel.org>

[ Upstream commit 8e4b1d2bc198e34b48fc7cc3a3c5a2fcb269e271 ]

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@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 4dfa9dd47223..fd37827eca5b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2498,7 +2498,6 @@ 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.
@@ -3315,6 +3314,7 @@ 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);
-- 
2.30.2