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

commit 2fcfe5951eb2e8440fc5e1dd6ea977336ff83a1d upstream.

scx_enable() uses double-checked locking to lazily initialize a static
kthread_worker pointer. The fast path reads helper locklessly:

    if (!READ_ONCE(helper)) {          // lockless read -- no helper_mutex

The write side initializes helper under helper_mutex, but previously
used a plain assignment:

        helper = kthread_run_worker(0, "scx_enable_helper");
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                 plain write -- KCSAN data race with READ_ONCE() above

Since READ_ONCE() on the fast path and the plain write on the
initialization path access the same variable without a common lock,
they constitute a data race. KCSAN requires that all sides of a
lock-free access use READ_ONCE()/WRITE_ONCE() consistently.

Use a temporary variable to stage the result of kthread_run_worker(),
and only WRITE_ONCE() into helper after confirming the pointer is
valid. This avoids a window where a concurrent caller on the fast path
could observe an ERR pointer via READ_ONCE(helper) before the error
check completes.

Fixes: b06ccbabe250 ("sched_ext: Fix starvation of scx_enable() under fair-class saturation")
Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index a4f9985b624af5c519a2854654481d122bf22be4..f7eeccbd893af38fd40920c4537cd7ac261530c3 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -5219,13 +5219,14 @@ 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");
-                       if (IS_ERR_OR_NULL(helper)) {
-                               helper = NULL;
+                       struct kthread_worker *w =
+                               kthread_run_worker(0, "scx_enable_helper");
+                       if (IS_ERR_OR_NULL(w)) {
                                mutex_unlock(&helper_mutex);
                                return -ENOMEM;
                        }
-                       sched_set_fifo(helper->task);
+                       sched_set_fifo(w->task);
+                       WRITE_ONCE(helper, w);
                }
                mutex_unlock(&helper_mutex);
        }