Re: [RFC PATCH v3 03/10] task_work, sched: Add a _locked variant to task_work_cancel()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 11, 2024 at 02:59:57PM +0200, Valentin Schneider wrote:
> Later commits will need to issue a task_work_cancel() from within the
> scheduler with the task's ->pi_lock held.
> 
> Add a _locked variant that expects p->pi_lock to be held. Expose it in a
> separate scheduler header file, as this really is a scheduler-only
> interface.
> 
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---
>  kernel/sched/task_work_sched.h | 14 +++++++
>  kernel/task_work.c             | 67 ++++++++++++++++++++++++++--------
>  2 files changed, 66 insertions(+), 15 deletions(-)
>  create mode 100644 kernel/sched/task_work_sched.h
> 
> diff --git a/kernel/sched/task_work_sched.h b/kernel/sched/task_work_sched.h
> new file mode 100644
> index 0000000000000..e235da456427f
> --- /dev/null
> +++ b/kernel/sched/task_work_sched.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Scheduler internal task_work methods
> + */
> +#ifndef _KERNEL_TASK_WORK_SCHED_H
> +#define _KERNEL_TASK_WORK_SCHED_H
> +
> +#include <linux/task_work.h>
> +#include <linux/sched.h>
> +
> +struct callback_head *
> +task_work_cancel_locked(struct task_struct *task, task_work_func_t func);
> +
> +#endif


Do we really need that exposed? Can't we squirrel that way in
kernel/sched/sched.h and forget about it?


> @@ -74,33 +76,20 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>  	return 0;
>  }
>  
> -/**
> - * task_work_cancel_match - cancel a pending work added by task_work_add()
> - * @task: the task which should execute the work
> - * @match: match function to call
> - * @data: data to be passed in to match function
> - *
> - * RETURNS:
> - * The found work or NULL if not found.
> - */
> -struct callback_head *
> -task_work_cancel_match(struct task_struct *task,
> +static struct callback_head *
> +task_work_cancel_match_locked(struct task_struct *task,
>  		       bool (*match)(struct callback_head *, void *data),
>  		       void *data)
>  {
>  	struct callback_head **pprev = &task->task_works;
>  	struct callback_head *work;
> -	unsigned long flags;
>  
> -	if (likely(!task_work_pending(task)))
> -		return NULL;
>  	/*
>  	 * If cmpxchg() fails we continue without updating pprev.
>  	 * Either we raced with task_work_add() which added the
>  	 * new entry before this work, we will find it again. Or
>  	 * we raced with task_work_run(), *pprev == NULL/exited.
>  	 */
> -	raw_spin_lock_irqsave(&task->pi_lock, flags);
>  	work = READ_ONCE(*pprev);
>  	while (work) {
>  		if (!match(work, data)) {
> @@ -109,6 +98,32 @@ task_work_cancel_match(struct task_struct *task,
>  		} else if (try_cmpxchg(pprev, &work, work->next))
>  			break;
>  	}
> +
> +	return work;
> +}

> @@ -136,6 +151,28 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
>  	return task_work_cancel_match(task, task_work_func_match, func);
>  }
>  
> +/**
> + * task_work_cancel - cancel a pending work added by task_work_add()
> + * @task: the task which should execute the work
> + * @func: identifies the work to remove
> + *
> + * Find the last queued pending work with ->func == @func and remove
> + * it from queue.
> + *
> + * RETURNS:
> + * The found work or NULL if not found.
> + */
> +struct callback_head *
> +task_work_cancel_locked(struct task_struct *task, task_work_func_t func)
> +{
> +	lockdep_assert_held(&task->pi_lock);

I'm thinking that lockde_assert wants to live in your _locked function
above.

> +	if (likely(!task_work_pending(task)))
> +		return NULL;
> +
> +	return task_work_cancel_match_locked(task, task_work_func_match, func);
> +}
> +
>  /**
>   * task_work_run - execute the works added by task_work_add()
>   *
> -- 
> 2.43.0
>