FAILED: patch "[PATCH] kernel/sched: Modify initial boot task idle setup" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x cff9b2332ab762b7e0586c793c431a8f2ea4db04
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2023100451-cough-encrypt-1d2e@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From cff9b2332ab762b7e0586c793c431a8f2ea4db04 Mon Sep 17 00:00:00 2001
From: "Liam R. Howlett" <[email protected]>
Date: Fri, 15 Sep 2023 13:44:44 -0400
Subject: [PATCH] kernel/sched: Modify initial boot task idle setup

Initial booting is setting the task flag to idle (PF_IDLE) by the call path sched_init() -> init_idle(). Having the task idle and calling call_rcu() in kernel/rcu/tiny.c means that TIF_NEED_RESCHED will be set. Subsequent calls to any cond_resched() will enable IRQs, potentially earlier than the IRQ setup has completed. Recent changes have caused just this scenario and IRQs have been enabled early.

This causes a warning later in start_kernel() as interrupts are enabled before they are fully set up.

Fix this issue by setting the PF_IDLE flag later in the boot sequence.

Although the boot task was marked as idle since (at least) d80e4fda576d, I am not sure that it is wrong to do so. The forced context-switch on idle task was introduced in the tiny_rcu update, so I'm going to claim this fixes 5f6130fa52ee.

index 2299a5cfbfb9..802551e0009b 100644
@@ -9269,7 +9269,7 @@ void __init init_idle(struct task_struct *idle, int cpu)
 	 * PF_KTHREAD should already be set at this point; regardless, make it
 	 * look like a proper per-CPU kthread.
 	 */
-	idle->flags |= PF_IDLE | PF_KTHREAD | PF_NO_SETAFFINITY;
+	idle->flags |= PF_KTHREAD | PF_NO_SETAFFINITY;
 	kthread_set_per_cpu(idle, cpu);
 
 #ifdef CONFIG_SMP

index 342f58a329f5..5007b25c5bc6 100644
@@ -373,6 +373,7 @@ EXPORT_SYMBOL_GPL(play_idle_precise);
 
 void cpu_startup_entry(enum cpuhp_state state)
 {
+	current->flags |= PF_IDLE;
 	arch_cpu_idle_prepare();
 	cpuhp_online_idle(state);
 	while (1)