linux-kernel - [PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task

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Date:   Wed, 16 Jan 2019 19:42:18 +0100
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Alan Stern <stern@...land.harvard.edu>
Subject: [PATCH] sched: Use READ_ONCE()/WRITE_ONCE() in task_cpu()/__set_task_cpu()

The smp_wmb() in move_queued_task() (c.f., __set_task_cpu()) pairs with
the composition of the dependency and the ACQUIRE in task_rq_lock():

	move_queued_task()		task_rq_lock()

	[S] ->on_rq = MIGRATING		[L] rq = task_rq()
	WMB (__set_task_cpu())		ACQUIRE (rq->lock);
	[S] ->cpu = new_cpu		[L] ->on_rq

where "[L] rq = task_rq()" is ordered before "ACQUIRE (rq->lock)" by an
address dependency and, in turn, "ACQUIRE (rq->lock)" is ordered before
"[L] ->on_rq" by the ACQUIRE itself.

Use READ_ONCE() to load ->cpu in task_rq() (c.f., task_cpu()) to honour
this address dependency between loads; also, mark the store to ->cpu in
__set_task_cpu() by using WRITE_ONCE() in order to tell the compiler to
not mess/tear this (synchronizing) memory access.

Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc: Alan Stern <stern@...land.harvard.edu>
---
 include/linux/sched.h | 4 ++--
 kernel/sched/sched.h  | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 224666226e87b..2bb02c9635bd8 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1753,9 +1753,9 @@ static __always_inline bool need_resched(void)
 static inline unsigned int task_cpu(const struct task_struct *p)
 {
 #ifdef CONFIG_THREAD_INFO_IN_TASK
-	return p->cpu;
+	return READ_ONCE(p->cpu);
 #else
-	return task_thread_info(p)->cpu;
+	return READ_ONCE(task_thread_info(p)->cpu);
 #endif
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index d04530bf251fe..270a3333589d2 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1460,9 +1460,9 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
 	 */
 	smp_wmb();
 #ifdef CONFIG_THREAD_INFO_IN_TASK
-	p->cpu = cpu;
+	WRITE_ONCE(p->cpu, cpu);
 #else
-	task_thread_info(p)->cpu = cpu;
+	WRITE_ONCE(task_thread_info(p)->cpu, cpu);
 #endif
 	p->wake_cpu = cpu;
 #endif
-- 
2.17.1