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: 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>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 351c0fe64c85d89cac35e9a1aa7579d4fa7ff40a..4112639c2a85fddc6b479a2eff703b65b13f4911 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -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
 }
 

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 32e06704565ec21fdc41ef3a5c0cfd9b80caa8c7..ec1b67a195cc1c00ecfac8d3586e13f941c1e96d 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -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);

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 99e2a7772d16ecd1174ad3e3a3a0654f9d86a4c2..c688ef5012e5b819e6ea3e85310f0ddccffdafe2 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -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;
 }
 
 /*