linux-kernel - Re: [Bug] Race condition between CPU hotplug off flow and __sched

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Date:   Thu, 19 May 2022 15:47:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jing-Ting Wu <jing-ting.wu@...iatek.com>
Cc:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>, tglx@...utronix.de,
        wsd_upstream@...iatek.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, Jonathan.JMChen@...iatek.com,
        "chris.redpath@....com" <chris.redpath@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Donnefort <vdonnefort@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [Bug] Race condition between CPU hotplug off flow and
 __sched_setscheduler()

On Thu, May 19, 2022 at 08:53:15PM +0800, Jing-Ting Wu wrote:
> Hi all
> 
> 
> There is a race condition between CPU hotplug off flow and
> __sched_setscheduler(), which will cause hang-up in CPU hotplug off
> flow.

How easy can you reproduce; does the below hack make it better?

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 95bac3b094b3..f18ee22b29bc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4763,20 +4763,30 @@ struct callback_head balance_push_callback = {
 	.func = (void (*)(struct callback_head *))balance_push,
 };
 
-static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
+static inline struct callback_head *
+__splice_balance_callbacks(struct rq *rq, bool foo)
 {
 	struct callback_head *head = rq->balance_callback;
 
 	lockdep_assert_rq_held(rq);
-	if (head)
-		rq->balance_callback = NULL;
+	if (head) {
+		if (foo && head == &balance_push_callback)
+			head = NULL;
+		else
+			rq->balance_callback = NULL;
+	}
 
 	return head;
 }
 
+static inline struct callback_head *splice_balance_callbacks(struct rq *rq)
+{
+	return __splice_balance_callbacks(rq, true);
+}
+
 static void __balance_callbacks(struct rq *rq)
 {
-	do_balance_callbacks(rq, splice_balance_callbacks(rq));
+	do_balance_callbacks(rq, __splice_balance_callbacks(rq, false));
 }
 
 static inline void balance_callbacks(struct rq *rq, struct callback_head *head)