From 1dde502587657045b267f179d7a1ecc7b8a1a265 Mon Sep 17 00:00:00 2001 From: David Carlier Date: Fri, 6 Mar 2026 04:50:55 +0000 Subject: [PATCH] sched_ext: Use READ_ONCE() for scx_slice_bypass_us in scx_bypass() Commit 0927780c90ce ("sched_ext: Use READ_ONCE() for lock-free reads of module param variables") annotated the plain reads of scx_slice_bypass_us and scx_bypass_lb_intv_us in bypass_lb_cpu(), but missed a third site in scx_bypass(): WRITE_ONCE(scx_slice_dfl, scx_slice_bypass_us * NSEC_PER_USEC); scx_slice_bypass_us is a module parameter writable via sysfs in process context through set_slice_us() -> param_set_uint_minmax(), which performs a plain store without holding bypass_lock. scx_bypass() reads the variable under bypass_lock, but since the writer does not take that lock, the two accesses are concurrent. WRITE_ONCE() only applies volatile semantics to the store of scx_slice_dfl -- the val expression containing scx_slice_bypass_us is evaluated as a plain read, providing no protection against concurrent writes. Wrap the read with READ_ONCE() to complete the annotation started by commit 0927780c90ce and make the access KCSAN-clean, consistent with the existing READ_ONCE(scx_slice_bypass_us) in bypass_lb_cpu(). Signed-off-by: David Carlier Signed-off-by: Tejun Heo --- kernel/sched/ext.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 10866bfb88bf..f323df7be180 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -4137,7 +4137,7 @@ static void scx_bypass(bool bypass) WARN_ON_ONCE(scx_bypass_depth <= 0); if (scx_bypass_depth != 1) goto unlock; - WRITE_ONCE(scx_slice_dfl, scx_slice_bypass_us * NSEC_PER_USEC); + WRITE_ONCE(scx_slice_dfl, READ_ONCE(scx_slice_bypass_us) * NSEC_PER_USEC); bypass_timestamp = ktime_get_ns(); if (sch) scx_add_event(sch, SCX_EV_BYPASS_ACTIVATE, 1);