[PATCH] sched_ext: Use READ_ONCE() for lock-free reads of module param variables

[zhidao su]

bypass_lb_cpu() reads scx_bypass_lb_intv_us and scx_slice_bypass_us
without holding any lock, in timer callback context where module
parameter writes via sysfs can happen concurrently:

    min_delta_us = scx_bypass_lb_intv_us / SCX_BYPASS_LB_MIN_DELTA_DIV;
                   ^^^^^^^^^^^^^^^^^^^^
                   plain read -- KCSAN data race

    if (delta < DIV_ROUND_UP(min_delta_us, scx_slice_bypass_us))
                                           ^^^^^^^^^^^^^^^^^
                                           plain read -- KCSAN data race

scx_bypass_lb_intv_us already uses READ_ONCE() in scx_bypass_lb_timerfn()
and scx_bypass() for its other lock-free read sites, leaving bypass_lb_cpu()
inconsistent. scx_slice_bypass_us has the same lock-free access pattern in
the same function.

Fix both plain reads by using READ_ONCE() to complete the concurrent
access annotation and make the code KCSAN-clean:

    min_delta_us = READ_ONCE(scx_bypass_lb_intv_us) / SCX_BYPASS_LB_MIN_DELTA_DIV;

    if (delta < DIV_ROUND_UP(min_delta_us, READ_ONCE(scx_slice_bypass_us)))

Signed-off-by: zhidao su <suzhidao@xxxxxxxxxx>
---
 kernel/sched/ext.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 1c3170846c84..9a1471ad5ae7 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4004,8 +4004,8 @@ static u32 bypass_lb_cpu(struct scx_sched *sch, struct rq *rq,
 	 * consider offloading iff the total queued duration is over the
 	 * threshold.
 	 */
-	min_delta_us = scx_bypass_lb_intv_us / SCX_BYPASS_LB_MIN_DELTA_DIV;
-	if (delta < DIV_ROUND_UP(min_delta_us, scx_slice_bypass_us))
+	min_delta_us = READ_ONCE(scx_bypass_lb_intv_us) / SCX_BYPASS_LB_MIN_DELTA_DIV;
+	if (delta < DIV_ROUND_UP(min_delta_us, READ_ONCE(scx_slice_bypass_us)))
 		return 0;
 
 	raw_spin_rq_lock_irq(rq);
-- 
2.43.0