Re: [Bug] Race condition between CPU hotplug off flow and __sched_setscheduler()

From: Peter Zijlstra <peterz@infradead.org>
To: Jing-Ting Wu <jing-ting.wu@mediatek.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>,
	tglx@linutronix.de, wsd_upstream@mediatek.com,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org, Jonathan.JMChen@mediatek.com,
	"chris.redpath@arm.com" <chris.redpath@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Vincent Donnefort <vdonnefort@gmail.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Christian Brauner <brauner@kernel.org>
Subject: Re: [Bug] Race condition between CPU hotplug off flow and __sched_setscheduler()
Date: Thu, 19 May 2022 15:47:06 +0200	[thread overview]
Message-ID: <20220519134706.GH2578@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <4a0aa13c99ffd6aea6426f83314aa2a91bc8933f.camel@mediatek.com>

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)

