linux-kernel - [PATCH v2 3/4] sched/rt: use put_task_struct_atomic

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Date:   Fri, 20 Jan 2023 12:02:41 -0300
From:   Wander Lairson Costa <wander@...hat.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Stafford Horne <shorne@...il.com>, Guo Ren <guoren@...nel.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Wander Lairson Costa <wander@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH v2 3/4] sched/rt: use put_task_struct_atomic_safe() to avoid potential splat

rto_push_irq_work_func() is called in hardirq context, and it calls
push_rt_task(), which calls put_task_struct().

If the kernel is compiled with PREEMPT_RT and put_task_struct() reaches
zero usage count, it triggers a splat because __put_task_struct()
indirectly acquires sleeping locks.

The put_task_struct() call pairs with an earlier get_task_struct(),
which makes the probability of the usage count reaches zero pretty
low. In any case, let's play safe and use the atomic safe version.

Signed-off-by: Wander Lairson Costa <wander@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/sched/rt.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index ed2a47e4ddae..30a4e9607bec 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2147,7 +2147,7 @@ static int push_rt_task(struct rq *rq, bool pull)
 		/*
 		 * Something has shifted, try again.
 		 */
-		put_task_struct(next_task);
+		put_task_struct_atomic_safe(next_task);
 		next_task = task;
 		goto retry;
 	}
@@ -2160,7 +2160,7 @@ static int push_rt_task(struct rq *rq, bool pull)
 
 	double_unlock_balance(rq, lowest_rq);
 out:
-	put_task_struct(next_task);
+	put_task_struct_atomic_safe(next_task);
 
 	return ret;
 }
-- 
2.39.0