On Mon, Jan 21, 2019 at 04:52:40PM +0100, Andrea Parri wrote:
> 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 <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: "Paul E. McKenney" <[email protected]>
> Cc: Alan Stern <[email protected]>
> Cc: Will Deacon <[email protected]>
ping
Andrea
> ---
> Changes in v2:
> - mark accesses to ->on_rq as well
> - update inline comment for task_rq_lock()
> - minor editing in the subject/changelog
>
> 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 d2f90fa924683..41212d725a0eb 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1754,9 +1754,9 @@ 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 a674c7db2f29d..d6e08faaa2843 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);
> @@ -915,7 +916,7 @@ 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 d04530bf251fe..425a5589e5f60 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
> @@ -1563,7 +1563,7 @@ 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;
> }
>
> /*
> --
> 2.17.1
>
