[PATCH 3/6] sched_ext: Return -EBUSY from a pre-init scx_bpf_sub_caps() read

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

scx_bpf_sub_caps() flags the scheduler for exit when the target's caps
storage isn't up yet. But the guard's own comment names a legitimate
way to get there: a scheduler reading its own caps from
ops.init_cids(), which runs before scx_alloc_pshards() publishes the
storage. An init_cids() implementation that starts by querying its
caps, a reasonable thing to do, kills the scheduler it is still
initializing.

Return -EBUSY instead: the caller sees a retryable "not ready yet"
and the comment's expected scenario stays survivable. This mirrors
how the cid path already handles the same class of pre-init window:
reads before the tables are published test the table pointer and
return -EINVAL instead of killing the scheduler.

Fixes: 86094b95efcf ("sched_ext: Add per-shard cap delegation for sub-schedulers")
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 kernel/sched/ext/sub.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index d2da4d2322f8..55e637e7a54a 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -3081,8 +3081,10 @@ __bpf_kfunc void scx_bpf_sub_revoke(u64 cgroup_id, u64 caps,
  * child - the literal granted set. A sched can read only itself or a direct
  * child.
  *
- * Return 0, -ENODEV if @cgroup_id names no direct child, or -EINVAL on bad
- * inputs.
+ * Return 0, -ENODEV if @cgroup_id names no direct child or the caller has
+ * no scheduler, -EINVAL on bad inputs, -EOPNOTSUPP on the cpu form, or
+ * -EBUSY if the target's caps storage is not up yet (expected from a
+ * self-read in ops.init_cids()).
  */
 __bpf_kfunc s32 scx_bpf_sub_caps(u64 cgroup_id, u64 caps, struct scx_cmask *out__arena,
 				 const struct bpf_prog_aux *aux)
@@ -3124,15 +3126,14 @@ __bpf_kfunc s32 scx_bpf_sub_caps(u64 cgroup_id, u64 caps, struct scx_cmask *out_
 
 	/*
 	 * The target's caps storage may not be set up yet (e.g. a self-read
-	 * during ops.init_cids()). Pairs with the publish in
-	 * scx_alloc_pshards(): a non-NULL pshard has every element set and the
-	 * acquire also orders the cid table reads below against it.
+	 * during ops.init_cids()); return -EBUSY instead of aborting the
+	 * scheduler. Pairs with the publish in scx_alloc_pshards(): a
+	 * non-NULL pshard has every element set and the acquire also orders
+	 * the cid table reads below against it.
 	 */
 	pshard = smp_load_acquire(&target->pshard);
-	if (unlikely(!pshard)) {
-		scx_error(sch, "scx_bpf_sub_caps() called before caps storage is initialized");
-		return -ENODEV;
-	}
+	if (unlikely(!pshard))
+		return -EBUSY;
 
 	ret = scx_cmask_ref_init(sch, out__arena, &ref);
 	if (ret) {
-- 
2.53.0

base-commit: 5b740a72b55c4267ba4d8010f17dde26b26f52d3