MIPS: kdump: Mark cpu back online before rebooting

The crash utility initializes cpu state by reading the system kernel
memory, which is copied into vmcore.

It is also natural to preserve the online state for CPUs at crash.

Failing to do so could make the analysis tool present info for only 1 CPU
by default, and unable to find panic task.
Signed-off-by: Dengcheng Zhu <dzhu@wavecomp.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20809/
Cc: Paul Burton <pburton@wavecomp.com>
Cc: "ralf@linux-mips.org" <ralf@linux-mips.org>
Cc: "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>
Cc: "rachel.mozes@intel.com" <rachel.mozes@intel.com>

arch/mips/kernel/machine_kexec.c

@@ -173,6 +173,16 @@ void kexec_reboot(void)
 {
 	void (*do_kexec)(void) __noreturn;
 
+	/*
+	 * We know we were online, and there will be no incoming IPIs at
+	 * this point. Mark online again before rebooting so that the crash
+	 * analysis tool will see us correctly.
+	 */
+	set_cpu_online(smp_processor_id(), true);
+
+	/* Ensure remote CPUs observe that we're online before rebooting. */
+	smp_mb__after_atomic();
+
 #ifdef CONFIG_SMP
 	if (smp_processor_id() > 0) {
 		/*