[PATCH sched_ext/for-7.4] sched_ext: Work around pahole 1.32 dropping scx_bpf_task_set_lazy_resched() from BTF

[Tejun Heo]

With clang and pahole 1.32, x86-64 vmlinux BTF lacks
scx_bpf_task_set_lazy_resched() and sched_ext fails to initialize:

  sched_ext: Failed to register kfunc sets (-22)

clang describes the lazy argument only after the prologue moves it into a
callee-saved register, which pahole 1.32 rejects as not being the argument
register. Released pahole is affected whatever the eventual fix. Keep the
argument on the stack with barrier_data() so that its location names no
register. OPTIMIZER_HIDE_VAR() would fix x86-64 but break arm64.

Fixes: f8e5a4e3f3be ("sched_ext: Add lazy preemption support")
Link: https://lore.kernel.org/r/20260925213619.2187751-1-yonghong.song@xxxxxxxxx
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 kernel/sched/ext/ext.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -10136,6 +10136,11 @@ __bpf_kfunc bool scx_bpf_task_set_lazy_r
 {
 	struct scx_sched *sch;
 
+	/*
+	 * clang's register locations for @lazy make pahole 1.32 drop this kfunc
+	 * from BTF. Keep @lazy in memory so its location names no register.
+	 */
+	barrier_data(&lazy);
 	guard(rcu)();
 	sch = scx_prog_sched(aux);
 	if (unlikely(!sch || !scx_task_on_sched(sch, p)))