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From:   Wander Lairson Costa <wander@redhat.com>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        Stafford Horne <shorne@gmail.com>, Guo Ren <guoren@kernel.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Wander Lairson Costa <wander@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
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        linux-kernel@vger.kernel.org (open list)
Subject: [PATCH v2 3/4] sched/rt: use put_task_struct_atomic_safe() to avoid potential splat
Date:   Fri, 20 Jan 2023 12:02:41 -0300
Message-Id: <20230120150246.20797-4-wander@redhat.com>
In-Reply-To: <20230120150246.20797-1-wander@redhat.com>
References: <20230120150246.20797-1-wander@redhat.com>
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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@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.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