[PATCH 1/2] sched_ext: Use READ_ONCE() for the read side of dsq->nr update

[zhidao su <soolaugust@gmail.com>]

From: zhidao su <suzhidao@xiaomi.com>

scx_bpf_dsq_nr_queued() reads dsq->nr via READ_ONCE() without holding
any lock, making dsq->nr a lock-free concurrently accessed variable.
However, dsq_mod_nr(), the sole writer of dsq->nr, only uses
WRITE_ONCE() on the write side without the matching READ_ONCE() on the
read side:

    WRITE_ONCE(dsq->nr, dsq->nr + delta);
                        ^^^^^^^
                        plain read -- KCSAN data race

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 read on the
right-hand side of WRITE_ONCE() leaves the pair incomplete and will
trigger KCSAN warnings.

Fix by using READ_ONCE() for the read side of the update:

    WRITE_ONCE(dsq->nr, READ_ONCE(dsq->nr) + delta);

This is consistent with scx_bpf_dsq_nr_queued() and makes the
concurrent access annotation complete and KCSAN-clean.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
---
 kernel/sched/ext.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 136b01950a62..718401f4ff2a 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -972,8 +972,12 @@ static bool scx_dsq_priq_less(struct rb_node *node_a,
 
 static void dsq_mod_nr(struct scx_dispatch_q *dsq, s32 delta)
 {
-	/* scx_bpf_dsq_nr_queued() reads ->nr without locking, use WRITE_ONCE() */
-	WRITE_ONCE(dsq->nr, dsq->nr + delta);
+	/*
+	 * scx_bpf_dsq_nr_queued() reads ->nr without locking. Use READ_ONCE()
+	 * on the read side and WRITE_ONCE() on the write side to properly
+	 * annotate the concurrent lockless access and avoid KCSAN warnings.
+	 */
+	WRITE_ONCE(dsq->nr, READ_ONCE(dsq->nr) + delta);
 }
 
 static void refill_task_slice_dfl(struct scx_sched *sch, struct task_struct *p)
-- 
2.43.0