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SubjectRe: [RFC][PATCH 12/17] sched: Also serialize ttwu_local() with p->pi_lock
(add Tejun)

On 12/24, Peter Zijlstra wrote:
>
> Since we now serialize ttwu() using p->pi_lock, we also need to
> serialize ttwu_local() using that, otherwise, once we drop the
> rq->lock from ttwu() it can race with ttwu_local().
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> kernel/sched.c | 28 +++++++++++++++++-----------
> 1 file changed, 17 insertions(+), 11 deletions(-)
>
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -2513,9 +2513,9 @@ static int try_to_wake_up(struct task_st
> * try_to_wake_up_local - try to wake up a local task with rq lock held
> * @p: the thread to be awakened
> *
> - * Put @p on the run-queue if it's not alredy there. The caller must
> + * Put @p on the run-queue if it's not alredy there. The caller must
> * ensure that this_rq() is locked, @p is bound to this_rq() and not
> - * the current task. this_rq() stays locked over invocation.
> + * the current task.
> */
> static void try_to_wake_up_local(struct task_struct *p)
> {
> @@ -2523,16 +2523,21 @@ static void try_to_wake_up_local(struct
>
> BUG_ON(rq != this_rq());
> BUG_ON(p == current);
> - lockdep_assert_held(&rq->lock);
> +
> + raw_spin_unlock(&rq->lock);
> + raw_spin_lock(&p->pi_lock);
> + raw_spin_lock(&rq->lock);

I _think_ this is safe, this worker can't change cpu afaics. But
probably Tejun can take a look, just in case.


>
> if (!(p->state & TASK_NORMAL))
> - return;
> + goto out;
>
> if (!p->on_rq)
> activate_task(rq, p, ENQUEUE_WAKEUP);
>
> ttwu_post_activation(p, rq, 0);
> ttwu_stat(rq, p, smp_processor_id(), 0);
> +out:
> + raw_spin_unlock(&p->pi_lock);
> }
>
> /**
> @@ -3925,6 +3930,7 @@ pick_next_task(struct rq *rq)
> */
> asmlinkage void __sched schedule(void)
> {
> + struct task_struct *to_wakeup = NULL;
> struct task_struct *prev, *next;
> unsigned long *switch_count;
> struct rq *rq;
> @@ -3958,21 +3964,21 @@ asmlinkage void __sched schedule(void)
> * task to maintain concurrency. If so, wake
> * up the task.
> */
> - if (prev->flags & PF_WQ_WORKER) {
> - struct task_struct *to_wakeup;
> -
> + if (prev->flags & PF_WQ_WORKER)
> to_wakeup = wq_worker_sleeping(prev, cpu);
> - if (to_wakeup)
> - try_to_wake_up_local(to_wakeup);
> - }
> deactivate_task(rq, prev, DEQUEUE_SLEEP);
> prev->on_rq = 0;
> }
> switch_count = &prev->nvcsw;
> }
>
> + /*
> + * All three: try_to_wake_up_local(), pre_schedule() and idle_balance()
> + * can drop rq->lock.
> + */
> + if (to_wakeup)
> + try_to_wake_up_local(to_wakeup);
> pre_schedule(rq, prev);
> -
> if (unlikely(!rq->nr_running))
> idle_balance(cpu, rq);
>
>
>



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