firmware: google: test spinlock on panic path to avoid lockups

Currently the gsmi driver registers a panic notifier as well as reboot and
die notifiers. The callbacks registered are called in atomic and very
limited context - for instance, panic disables preemption and local IRQs,
also all secondary CPUs (not executing the panic path) are shutdown.

With that said, taking a spinlock in this scenario is a dangerous
invitation for lockup scenarios. So, fix that by checking if the spinlock
is free to acquire in the panic notifier callback - if not, bail-out and
avoid a potential hang.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 74c5b31c6618 ("driver: Google EFI SMI")
Signed-off-by: Guilherme G. Piccoli <[e-mail]>
Reviewed-by: Evan Green <[e-mail]>
Cc: Ard Biesheuvel <[e-mail]>
Cc: David Gow <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Julius Werner <[e-mail]>
Cc: Petr Mladek <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/firmware/google/gsmi.c

···
 static int gsmi_panic_callback(struct notifier_block *nb,
 			       unsigned long reason, void *arg)
 {
+
+	/*
+	 * Panic callbacks are executed with all other CPUs stopped,
+	 * so we must not attempt to spin waiting for gsmi_dev.lock
+	 * to be released.
+	 */
+	if (spin_is_locked(&gsmi_dev.lock))
+		return NOTIFY_DONE;
+
 	gsmi_shutdown_reason(GSMI_SHUTDOWN_PANIC);
 	return NOTIFY_DONE;
 }