linux-kernel - Re: [PATCH] sched: Recheck the rt task's on rq state after double_lock

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Message-ID: <xhsmhqzuxlolh.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Mon, 20 Oct 2025 14:55:22 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Tengfei Fan <tengfei.fan@....qualcomm.com>, Ingo Molnar
 <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli
 <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
 <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
 <mgorman@...e.de>, linux-arm-msm@...r.kernel.org
Cc: kernel@....qualcomm.com, linux-kernel@...r.kernel.org, Tengfei Fan
 <tengfei.fan@....qualcomm.com>
Subject: Re: [PATCH] sched: Recheck the rt task's on rq state after
 double_lock_balance()

On 09/10/25 00:23, Tengfei Fan wrote:
> Recheck whether next_task is still in the runqueue of this_rq after
> locking this_rq and lowest_rq via double_lock_balance() in
> push_rt_task(). This is necessary because double_lock_balance() first
> releases this_rq->lock and then attempts to acquire both this_rq->lock
> and lowest_rq->lock, during which next_task may have already been
> removed from this_rq's runqueue, leading to a double dequeue issue.
>
> The double dequeue issue can occur in the following scenario:
> 1. Core0 call stack:
>         autoremove_wake_function
>         default_wake_function
>         try_to_wake_up
>         ttwu_do_activate
>         task_woken_rt
>         push_rt_task
>         move_queued_task_locked
>         dequeue_task
>         __wake_up
>
> 2. Execution flow on Core0, Core1 and Core2(Core0, Core1 and Core2 are
>    contending for Core1's rq->lock):
>    - Core1: enqueue next_task on Core1
>    - Core0: lock Core1's rq->lock
>             next_task = pick_next_pushable_task()
>             unlock Core1's rq->lock via double_lock_balance()
>    - Core1: lock Core1's rq->lock
>             next_task = pick_next_task()
>             unlock Core1's rq->lock
>    - Core2: lock Core1's rq->lock in migration thread
>    - Core1: running next_task
>    - Core2: unlock Core1's rq->lock
>    - Core1: lock Core1's rq->lock
>             switches out and dequeue next_task
>             unlock Core1's rq->lock
>    - Core0: relock Core1's rq->lock from double_lock_balance()
>             try to relock Core1's rq->lock from double_lock_balance()
>             but next_task has been dequeued from Core1, causing the issue
>
> Signed-off-by: Tengfei Fan <tengfei.fan@....qualcomm.com>
> ---
>  kernel/sched/rt.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 7936d4333731..b4e44317a5de 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2037,6 +2037,14 @@ static int push_rt_task(struct rq *rq, bool pull)
>  		goto retry;
>  	}
>  
> +	/* Within find_lock_lowest_rq(), it's possible to first unlock the
> +	 * rq->lock of the runqueue containing next_task, and the re->lock
> +	 * it. During this window, the state of next_task might have change.
> +	 */
> +	if (unlikely(rq != task_rq(next_task) ||
> +		     !task_on_rq_queued(next_task)))
> +		goto out;
> +

Isn't this already covered by find_lock_lowest_rq()?

if @next_task migrates during the double_lock_balance(), we'll see that
it's no longer the next highest priority pushable task of its original rq
(it won't be in that pushable list at all actually):

  static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
  {
          [...]
          if (double_lock_balance(rq, lowest_rq)) {
                  if (unlikely(is_migration_disabled(task) ||
                               !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) ||
                               task != pick_next_pushable_task(rq))) {

                          double_unlock_balance(rq, lowest_rq);
                          lowest_rq = NULL;
                          break;
                  }
          }
  }                

Plus:

  static int push_rt_task(struct rq *rq, bool pull)
  {
          [...]
          if (!lowest_rq) {
                  struct task_struct *task;
                  task = pick_next_pushable_task(rq);
                  [...]
                  put_task_struct(next_task);
                  next_task = task;
                  goto retry;
          }
  }        

AFAICT in the scenario you described, we'd just retry with another next
pushable task.

>  	move_queued_task_locked(rq, lowest_rq, next_task);
>  	resched_curr(lowest_rq);
>  	ret = 1;
>
> ---
> base-commit: 7c3ba4249a3604477ea9c077e10089ba7ddcaa03
> change-id: 20251008-recheck_rt_task_enqueue_state-e159aa6a2749
>
> Best regards,
> -- 
> Tengfei Fan <tengfei.fan@....qualcomm.com>