Commit c546951d authored by Andrea Parri's avatar Andrea Parri Committed by Ingo Molnar
Browse files

sched/core: Use READ_ONCE()/WRITE_ONCE() in move_queued_task()/task_rq_lock() 



move_queued_task() synchronizes with task_rq_lock() as follows:

	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 honor
this address dependency.  Also, mark the accesses to ->cpu and ->on_rq
with READ_ONCE()/WRITE_ONCE() to comply with the LKMM.

Signed-off-by: default avatarAndrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: https://lkml.kernel.org/r/20190121155240.27173-1-andrea.parri@amarulasolutions.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 1ca4fa3a

include/linux/sched.h

+2 −2
Original line number Diff line number Diff line
@@ -1745,9 +1745,9 @@ 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

}

kernel/sched/core.c

+5 −4
Original line number Diff line number Diff line
@@ -107,11 +107,12 @@ struct rq *task_rq_lock(struct task_struct *p, struct rq_flags *rf) 
		 *					[L] ->on_rq

		 *	RELEASE (rq->lock)

		 *

		 * If we observe the old CPU in task_rq_lock, the acquire of

		 * If we observe the old CPU in task_rq_lock(), the acquire of

		 * the old rq->lock will fully serialize against the stores.

		 *

		 * If we observe the new CPU in task_rq_lock, the acquire will

		 * pair with the WMB to ensure we must then also see migrating.

		 * If we observe the new CPU in task_rq_lock(), the address

		 * dependency headed by '[L] rq = task_rq()' and the acquire

		 * will pair with the WMB to ensure we then also see migrating.

		 */

		if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) {

			rq_pin_lock(rq, rf);
@@ -916,7 +917,7 @@ static struct rq *move_queued_task(struct rq *rq, struct rq_flags *rf, 
{

	lockdep_assert_held(&rq->lock);



	p->on_rq = TASK_ON_RQ_MIGRATING;

	WRITE_ONCE(p->on_rq, TASK_ON_RQ_MIGRATING);

	dequeue_task(rq, p, DEQUEUE_NOCLOCK);

	set_task_cpu(p, new_cpu);

	rq_unlock(rq, rf);

kernel/sched/sched.h

+3 −3
Original line number Diff line number Diff line
@@ -1479,9 +1479,9 @@ 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
@@ -1582,7 +1582,7 @@ static inline int task_on_rq_queued(struct task_struct *p) 


static inline int task_on_rq_migrating(struct task_struct *p)

{

	return p->on_rq == TASK_ON_RQ_MIGRATING;

	return READ_ONCE(p->on_rq) == TASK_ON_RQ_MIGRATING;

}



/*

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