Re: [PATCH] sched/deadline: Use task_on_rq_migrating() helper

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 08, 2026 at 02:53:07PM +0800, luoliang@kylinos.cn wrote:
> From: Liang Luo <luoliang@kylinos.cn>
> 
> Replace the open-coded "p->on_rq == TASK_ON_RQ_MIGRATING" comparisons
> in enqueue_task_dl() and dequeue_task_dl() with the existing
> task_on_rq_migrating() helper, consistent with the rest of the
> scheduler code.
> 
> The helper uses READ_ONCE() when loading ->on_rq, which was added by
> commit c546951d9c93 ("sched/core: Use READ_ONCE()/WRITE_ONCE() in
> move_queued_task()/task_rq_lock()") for LKMM compliance.  As documented
> in that commit, move_queued_task() synchronizes with task_rq_lock():
> 
>   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
> 
> The READ_ONCE() on the ->on_rq load ensures proper pairing with the
> WRITE_ONCE() on the writer side.  The open-coded checks in deadline.c
> lack this annotation.
> 
> No functional change.

This is completely irrelevant here. p->on_rq is set with rq->lock held,
and tasks are {en,de}queued with rq->lock held. There is no concurrency.