[PATCH] sched_ext: Use WRITE_ONCE() for the write side of scx_enable helper pointer

[zhidao su]

scx_enable() uses double-checked locking to lazily initialize a static
kthread_worker pointer:

    if (!READ_ONCE(helper)) {
        mutex_lock(&helper_mutex);
        if (!helper) {
            helper = kthread_run_worker(0, "scx_enable_helper");
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                     plain write -- KCSAN data race

The outer READ_ONCE() annotates the lockless fast-path read, but the
write side uses a plain assignment without the matching WRITE_ONCE().
The KCSAN documentation requires that if one accessor uses READ_ONCE()
or WRITE_ONCE() on a variable to annotate lock-free access, all other
accesses must also use the appropriate accessor. A plain write leaves
the pair incomplete and will trigger KCSAN warnings.

The error path also has the same issue:

            helper = NULL;
            ^^^^^^^^^^
            plain write -- KCSAN data race

Fix both plain writes by using WRITE_ONCE() to complete the concurrent
access annotation and make the code KCSAN-clean.

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 9a1471ad5ae7..c4ccd685259f 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5355,9 +5355,9 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	if (!READ_ONCE(helper)) {
 		mutex_lock(&helper_mutex);
 		if (!helper) {
-			helper = kthread_run_worker(0, "scx_enable_helper");
+			WRITE_ONCE(helper, kthread_run_worker(0, "scx_enable_helper"));
 			if (IS_ERR_OR_NULL(helper)) {
-				helper = NULL;
+				WRITE_ONCE(helper, NULL);
 				mutex_unlock(&helper_mutex);
 				return -ENOMEM;
 			}
-- 
2.43.0