powerpc: Offline CPU in stop_this_cpu()

printk_safe_flush_on_panic() has special lock breaking code for the case
where we panic()ed with the console lock held. It relies on panic IPI
causing other CPUs to mark themselves offline.

Do as most other architectures do.

This effectively reverts commit de6e5d38 ("powerpc: smp_send_stop do
not offline stopped CPUs"), unfortunately it may result in some false
positive warnings, but the alternative is more situations where we can
crash without getting messages out.

Fixes: de6e5d38 ("powerpc: smp_send_stop do not offline stopped CPUs")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210623041245.865134-1-npiggin@gmail.com

arch/powerpc/kernel/smp.c

@@ -619,6 +619,8 @@ static void nmi_stop_this_cpu(struct pt_regs *regs)
 	/*
 	 * IRQs are already hard disabled by the smp_handle_nmi_ipi.
 	 */
+	set_cpu_online(smp_processor_id(), false);
+
 	spin_begin();
 	while (1)
 		spin_cpu_relax();
@@ -634,6 +636,15 @@ void smp_send_stop(void)
 static void stop_this_cpu(void *dummy)
 {
 	hard_irq_disable();
+
+	/*
+	 * Offlining CPUs in stop_this_cpu can result in scheduler warnings,
+	 * (see commit de6e5d38417e), but printk_safe_flush_on_panic() wants
+	 * to know other CPUs are offline before it breaks locks to flush
+	 * printk buffers, in case we panic()ed while holding the lock.
+	 */
+	set_cpu_online(smp_processor_id(), false);
+
 	spin_begin();
 	while (1)
 		spin_cpu_relax();