sched_ext: Warn on task-based SCX op recursion

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 <[email protected]>
Acked-by: Andrea Righi <[email protected]>

Author: htejun

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);		\