Re: [PATCH 10/10] sched_ext: Warn on task-based SCX op recursion

[Andrea Righi]

On Thu, Apr 09, 2026 at 08:30:46PM -1000, Tejun Heo wrote:
> The kf_tasks[] design assumes task-based SCX ops don't nest - if they
> did, kf_tasks[0] would get clobbered. The old scx_kf_allow() WARN_ONCE
> caught invalid nesting via kf_mask, but that machinery is gone now.
> 
> Add a WARN_ON_ONCE(current->scx.kf_tasks[0]) at the top of each
> SCX_CALL_OP_TASK*() macro. Checking kf_tasks[0] alone is sufficient: all
> three variants (SCX_CALL_OP_TASK, SCX_CALL_OP_TASK_RET,
> SCX_CALL_OP_2TASKS_RET) write to kf_tasks[0], so a non-NULL value at
> entry to any of the three means re-entry from somewhere in the family.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>

Acked-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> ---
>  kernel/sched/ext.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 27091ae075a3..99760d1fbbd4 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -502,10 +502,13 @@ do {										\
>   * held by try_to_wake_up() with rq tracking via scx_rq.in_select_cpu. So if
>   * kf_tasks[] is set, @p's scheduler-protected fields are stable.
>   *
> - * These macros only work for non-nesting ops since kf_tasks[] is not stacked.
> + * kf_tasks[] can not stack, so task-based SCX ops must not nest. The
> + * WARN_ON_ONCE() in each macro catches a re-entry of any of the three variants
> + * while a previous one is still in progress.
>   */
>  #define SCX_CALL_OP_TASK(sch, op, rq, task, args...)				\
>  do {										\
> +	WARN_ON_ONCE(current->scx.kf_tasks[0]);					\
>  	current->scx.kf_tasks[0] = task;					\
>  	SCX_CALL_OP((sch), op, rq, task, ##args);				\
>  	current->scx.kf_tasks[0] = NULL;					\
> @@ -514,6 +517,7 @@ do {										\
>  #define SCX_CALL_OP_TASK_RET(sch, op, rq, task, args...)			\
>  ({										\
>  	__typeof__((sch)->ops.op(task, ##args)) __ret;				\
> +	WARN_ON_ONCE(current->scx.kf_tasks[0]);					\
>  	current->scx.kf_tasks[0] = task;					\
>  	__ret = SCX_CALL_OP_RET((sch), op, rq, task, ##args);			\
>  	current->scx.kf_tasks[0] = NULL;					\
> @@ -523,6 +527,7 @@ do {										\
>  #define SCX_CALL_OP_2TASKS_RET(sch, op, rq, task0, task1, args...)		\
>  ({										\
>  	__typeof__((sch)->ops.op(task0, task1, ##args)) __ret;			\
> +	WARN_ON_ONCE(current->scx.kf_tasks[0]);					\
>  	current->scx.kf_tasks[0] = task0;					\
>  	current->scx.kf_tasks[1] = task1;					\
>  	__ret = SCX_CALL_OP_RET((sch), op, rq, task0, task1, ##args);		\
> -- 
> 2.53.0
>