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

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.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index c56de568ed94b2e1fe674bb9071955e161f59625..10866bfb88bf55c9443e397d3d8d0d5be2a73015 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3908,8 +3908,8 @@ 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);