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

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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: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Peter 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: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andrea Parri 2019-01-21 16:52:40 +01:00 committed by Ingo Molnar
parent 1ca4fa3ab6
commit c546951d9c
3 changed files with 10 additions and 9 deletions
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4
include/linux/sched.h
View File

@ -1745,9 +1745,9 @@ static __always_inline bool need_resched(void)
static inline unsigned int task_cpu(const struct task_struct *p) static inline unsigned int task_cpu(const struct task_struct *p)
{ {
#ifdef CONFIG_THREAD_INFO_IN_TASK #ifdef CONFIG_THREAD_INFO_IN_TASK
return p->cpu; return READ_ONCE(p->cpu);
#else #else
return task_thread_info(p)->cpu; return READ_ONCE(task_thread_info(p)->cpu);
#endif #endif
} }

9
kernel/sched/core.c
View File

@ -107,11 +107,12 @@ struct rq *task_rq_lock(struct task_struct *p, struct rq_flags *rf)
* [L] ->on_rq * [L] ->on_rq
* RELEASE (rq->lock) * 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. * the old rq->lock will fully serialize against the stores.
* *
* If we observe the new CPU in task_rq_lock, the acquire will * If we observe the new CPU in task_rq_lock(), the address
* pair with the WMB to ensure we must then also see migrating. * 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))) { if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) {
rq_pin_lock(rq, rf); 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); 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); dequeue_task(rq, p, DEQUEUE_NOCLOCK);
set_task_cpu(p, new_cpu); set_task_cpu(p, new_cpu);
rq_unlock(rq, rf); rq_unlock(rq, rf);

6
kernel/sched/sched.h
View File

@ -1479,9 +1479,9 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
*/ */
smp_wmb(); smp_wmb();
#ifdef CONFIG_THREAD_INFO_IN_TASK #ifdef CONFIG_THREAD_INFO_IN_TASK
p->cpu = cpu; WRITE_ONCE(p->cpu, cpu);
#else #else
task_thread_info(p)->cpu = cpu; WRITE_ONCE(task_thread_info(p)->cpu, cpu);
#endif #endif
p->wake_cpu = cpu; p->wake_cpu = cpu;
#endif #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) 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;
} }
/* /*