debug: track and print last unloaded module in the oops trace

Based on a suggestion from Andi:

 In various cases, the unload of a module may leave some bad state around
 that causes a kernel crash AFTER a module is unloaded; and it's then hard
 to find which module caused that.

This patch tracks the last unloaded module, and prints this as part of the
module list in the oops trace.

Right now, only the last 1 module is tracked; I expect that this is enough
for the vast majority of cases where this information matters; if it turns
out that tracking more is important, we can always extend it to that.

[ mingo@elte.hu: build fix ]
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/module.c

@@ -496,6 +496,8 @@ static struct module_attribute modinfo_##field = {                    \
 MODINFO_ATTR(version);
 MODINFO_ATTR(srcversion);
 
+static char last_unloaded_module[MODULE_NAME_LEN+1];
+
 #ifdef CONFIG_MODULE_UNLOAD
 /* Init the unload section of the module. */
 static void module_unload_init(struct module *mod)
@@ -719,6 +721,8 @@ sys_delete_module(const char __user *name_user, unsigned int flags)
 		mod->exit();
 		mutex_lock(&module_mutex);
 	}
+	/* Store the name of the last unloaded module for diagnostic purposes */
+	sprintf(last_unloaded_module, mod->name);
 	free_module(mod);
 
  out:
@@ -2503,6 +2507,8 @@ void print_modules(void)
 	printk("Modules linked in:");
 	list_for_each_entry(mod, &modules, list)
 		printk(" %s%s", mod->name, module_flags(mod, buf));
+	if (last_unloaded_module[0])
+		printk(" [last unloaded: %s]", last_unloaded_module);
 	printk("\n");
 }