From: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx> [ Upstream commit c546951d9c9300065bad253ecdf1ac59ce9d06c8 ] 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@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Mike Galbraith <efault@xxxxxx> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Will Deacon <will.deacon@xxxxxxx> Link: https://lkml.kernel.org/r/20190121155240.27173-1-andrea.parri@xxxxxxxxxxxxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- include/linux/sched.h | 4 ++-- kernel/sched/core.c | 9 +++++---- kernel/sched/sched.h | 6 +++--- 3 files changed, 10 insertions(+), 9 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index 4abb5bd74b04..5dc024e28397 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1737,9 +1737,9 @@ 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 152a0b0c91bb..9a4f57d7e931 100644 --- 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); @@ -910,7 +911,7 @@ 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 b63172288f7b..4c7a837d7c14 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1331,9 +1331,9 @@ 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 @@ -1434,7 +1434,7 @@ 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; } /* -- 2.19.1
