driver-core: add driver module asynchronous probe support

Some init systems may wish to express the desire to have device drivers
run their probe() code asynchronously. This implements support for this
and allows userspace to request async probe as a preference through a
generic shared device driver module parameter, async_probe.

Implementation for async probe is supported through a module parameter
given that since synchronous probe has been prevalent for years some
userspace might exist which relies on the fact that the device driver
will probe synchronously and the assumption that devices it provides
will be immediately available after this.

Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/kernel-parameters.txt

@@ -943,6 +943,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			auto	selects the default scheme, which automatically
 				enables eagerfpu restore for xsaveopt.
 
+	module.async_probe [KNL]
+			Enable asynchronous probe on this module.
+
 	early_ioremap_debug [KNL]
 			Enable debug messages in early_ioremap support. This
 			is useful for tracking down temporary early mappings

drivers/base/dd.c

@@ -419,7 +419,13 @@ int driver_probe_device(struct device_driver *drv, struct device *dev)
 
 bool driver_allows_async_probing(struct device_driver *drv)
 {
-	return drv->probe_type == PROBE_PREFER_ASYNCHRONOUS;
+	if (drv->probe_type == PROBE_PREFER_ASYNCHRONOUS)
+		return true;
+
+	if (drv->owner && drv->owner->async_probe_requested)
+		return true;
+
+	return false;
 }
 
 struct device_attach_data {

include/linux/device.h

@@ -201,10 +201,12 @@ extern struct klist *bus_get_device_klist(struct bus_type *bus);
  *	respective probe routines. This tells the core what to
  *	expect and prefer.
  *
- * @PROBE_SYNCHRONOUS: Default. Drivers expect their probe routines
+ * @PROBE_DEFAULT_STRATEGY: Drivers expect their probe routines
  *	to run synchronously with driver and device registration
  *	(with the exception of -EPROBE_DEFER handling - re-probing
- *	always ends up being done asynchronously).
+ *	always ends up being done asynchronously) unless user
+ *	explicitly requested asynchronous probing via module
+ *	parameter.
  * @PROBE_PREFER_ASYNCHRONOUS: Drivers for "slow" devices which
  *	probing order is not essential for booting the system may
  *	opt into executing their probes asynchronously.
@@ -216,7 +218,7 @@ extern struct klist *bus_get_device_klist(struct bus_type *bus);
  * drivers.
  */
 enum probe_type {
-	PROBE_SYNCHRONOUS,
+	PROBE_DEFAULT_STRATEGY,
 	PROBE_PREFER_ASYNCHRONOUS,
 };
 

include/linux/module.h

@@ -257,6 +257,8 @@ struct module {
 	bool sig_ok;
 #endif
 
+	bool async_probe_requested;
+
 	/* symbols that will be GPL-only in the near future. */
 	const struct kernel_symbol *gpl_future_syms;
 	const unsigned long *gpl_future_crcs;

kernel/module.c

@@ -3107,7 +3107,7 @@ static noinline int do_init_module(struct module *mod)
 	 *
 	 * http://thread.gmane.org/gmane.linux.kernel/1420814
 	 */
-	if (current->flags & PF_USED_ASYNC)
+	if (!mod->async_probe_requested && (current->flags & PF_USED_ASYNC))
 		async_synchronize_full();
 
 	mutex_lock(&module_mutex);
@@ -3240,8 +3240,16 @@ out:
 static int unknown_module_param_cb(char *param, char *val, const char *modname,
 				   void *arg)
 {
+	struct module *mod = arg;
+	int ret;
+
+	if (strcmp(param, "async_probe") == 0) {
+		mod->async_probe_requested = true;
+		return 0;
+	}
+
 	/* Check for magic 'dyndbg' arg */
-	int ret = ddebug_dyndbg_module_param_cb(param, val, modname);
+	ret = ddebug_dyndbg_module_param_cb(param, val, modname);
 	if (ret != 0)
 		pr_warn("%s: unknown parameter '%s' ignored\n", modname, param);
 	return 0;