[PATCH v2 5/6] sched/core: Pause KCOV in try_to_wake_up()

[Karl Mehltretter <kmehltretter@gmail.com>]

try_to_wake_up() is uninstrumented, but it calls instrumented helpers
such as kthread_is_per_cpu(), CPU capacity helpers and SCHED_HRTICK
arming. They can record into current while in_task() is true.

CONFIG_KCOV_SELFTEST exposes this under PREEMPT_RT. The interrupt
selftest fails on x86_64 in kthread_is_per_cpu(): RT runs the timer
softirq in a thread, so the wakeup runs in task context during the
selftest's spin. The same helpers leak into non-RT syscall wakeups such
as a pipe write waking a reader.

Pause the wakeup body with the kcov_pause guard. Wrapping only
select_task_rq() would miss SCHED_HRTICK arming during enqueue.

Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Notes:
    v2:
     - take guard(kcov_pause)()
     - describe the paused wakeup body precisely

 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e49f0bfa0ca73..e863fac02e38f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4253,6 +4253,9 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	guard(preempt)();
 	int cpu, success = 0;
 
+	/* Instrumented callees would leak coverage into current. */
+	guard(kcov_pause)();
+
 	wake_flags |= WF_TTWU;
 
 	if (p == current) {
-- 
2.53.0