linux-kernel - [PATCH tip/core/rcu 05/18] rcu: Set rcutree.kthread

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Date:   Tue, 26 Mar 2019 16:13:01 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Liu Song <fishland@...yun.com>,
        Liu Song <liu.song11@....com.cn>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH tip/core/rcu 05/18] rcu: Set rcutree.kthread_prio sysfs access to read-only

From: Liu Song <fishland@...yun.com>

The rcutree.kthread_prio kernel-boot parameter is used to set the
priority for boost (rcub), per-CPU (rcuc), and grace-period (rcu_preempt
or rcu_sched) kthreads.  It is also used by rcutorture to check whether
it is possible to meaningfully test RCU priority boosting.  However,
all of these cases will either ignore or be confused by any post-boot
changes to rcutree.kthread_prio.

Note that the user really can change the priorities of all of these
kthreads using chrt, given sufficient privileges.  Therefore, the
read-write nature of sysfs access to rcutree.kthread_prio is thus at
best an attractive nuisance.

This commit therefore changes sysfs access to rcutree.kthread_prio to
be read-only.

Signed-off-by: Liu Song <liu.song11@....com.cn>
Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 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 2f78a115d34c..296131450414 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -149,7 +149,7 @@ static void sync_sched_exp_online_cleanup(int cpu);

 /* rcuc/rcub kthread realtime priority */
 static int kthread_prio = IS_ENABLED(CONFIG_RCU_BOOST) ? 1 : 0;
-module_param(kthread_prio, int, 0644);
+module_param(kthread_prio, int, 0444);

 /* Delay in jiffies for grace-period initialization delays, debug only. */

-- 
2.17.1