sched_ext: Fix is_bpf_migration_disabled() false negative on non-PREEMPT_RCU

Since commit 8e4f0b1 ("bpf: use rcu_read_lock_dont_migrate() for
trampoline.c"), the BPF prolog (__bpf_prog_enter) calls migrate_disable()
only when CONFIG_PREEMPT_RCU is enabled, via rcu_read_lock_dont_migrate().
Without CONFIG_PREEMPT_RCU, the prolog never touches migration_disabled,
so migration_disabled == 1 always means the task is truly
migration-disabled regardless of whether it is the current task.

The old unconditional p == current check was a false negative in this
case, potentially allowing a migration-disabled task to be dispatched to
a remote CPU and triggering scx_error in task_can_run_on_remote_rq().

Only apply the p == current disambiguation when CONFIG_PREEMPT_RCU is
enabled, where the ambiguity with the BPF prolog still exists.

Fixes: 8e4f0b1 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c")
Cc: stable@vger.kernel.org # v6.18+
Link: https://lore.kernel.org/lkml/20250821090609.42508-8-dongml2@chinatelecom.cn/
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: multics69

kernel/sched/ext_idle.c

@@ -860,25 +860,32 @@ static bool check_builtin_idle_enabled(struct scx_sched *sch)
  * code.
  *
  * We can't simply check whether @p->migration_disabled is set in a
- * sched_ext callback, because migration is always disabled for the current
- * task while running BPF code.
+ * sched_ext callback, because the BPF prolog (__bpf_prog_enter) may disable
+ * migration for the current task while running BPF code.
  *
- * The prolog (__bpf_prog_enter) and epilog (__bpf_prog_exit) respectively
- * disable and re-enable migration. For this reason, the current task
- * inside a sched_ext callback is always a migration-disabled task.
+ * Since the BPF prolog calls migrate_disable() only when CONFIG_PREEMPT_RCU
+ * is enabled (via rcu_read_lock_dont_migrate()), migration_disabled == 1 for
+ * the current task is ambiguous only in that case: it could be from the BPF
+ * prolog rather than a real migrate_disable() call.
  *
- * Therefore, when @p->migration_disabled == 1, check whether @p is the
- * current task or not: if it is, then migration was not disabled before
- * entering the callback, otherwise migration was disabled.
+ * Without CONFIG_PREEMPT_RCU, the BPF prolog never calls migrate_disable(),
+ * so migration_disabled == 1 always means the task is truly
+ * migration-disabled.
+ *
+ * Therefore, when migration_disabled == 1 and CONFIG_PREEMPT_RCU is enabled,
+ * check whether @p is the current task or not: if it is, then migration was
+ * not disabled before entering the callback, otherwise migration was disabled.
  *
  * Returns true if @p is migration-disabled, false otherwise.
  */
 static bool is_bpf_migration_disabled(const struct task_struct *p)
 {
-	if (p->migration_disabled == 1)
-		return p != current;
-	else
-		return p->migration_disabled;
+	if (p->migration_disabled == 1) {
+		if (IS_ENABLED(CONFIG_PREEMPT_RCU))
+			return p != current;
+		return true;
+	}
+	return p->migration_disabled;
 }
 
 static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p,