From 65e5f2cdd458278b2fff8d23cac133396e339c07 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Mon, 20 Jan 2020 17:23:03 +0000
Subject: [PATCH] x86: Wait for curthread to be set up as an indicator that the
 boot stack is no longer used.

pc_curthread is set by cpu_switch after it stopped using the old
thread (or boot) stack.  This makes the smp_after_idle_runnable()
function not dependent on the internals of the scheduler operations.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D23276
---
 sys/x86/x86/mp_x86.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/sys/x86/x86/mp_x86.c b/sys/x86/x86/mp_x86.c
index efe221c86f2..e4354c08f06 100644
--- a/sys/x86/x86/mp_x86.c
+++ b/sys/x86/x86/mp_x86.c
@@ -1092,13 +1092,12 @@ init_secondary_tail(void)
 static void
 smp_after_idle_runnable(void *arg __unused)
 {
-	struct thread *idle_td;
+	struct pcpu *pc;
 	int cpu;
 
 	for (cpu = 1; cpu < mp_ncpus; cpu++) {
-		idle_td = pcpu_find(cpu)->pc_idlethread;
-		while (atomic_load_int(&idle_td->td_lastcpu) == NOCPU &&
-		    atomic_load_int(&idle_td->td_oncpu) == NOCPU)
+		pc = pcpu_find(cpu);
+		while (atomic_load_ptr(&pc->pc_curthread) == (uintptr_t)NULL)
 			cpu_spinwait();
 		kmem_free((vm_offset_t)bootstacks[cpu], kstack_pages *
 		    PAGE_SIZE);