module: create a request_module_nowait()

There seems to be a common pattern in the kernel where drivers want to
call request_module() from inside a module_init() function. Currently
this would deadlock.

As a result, several drivers go through hoops like scheduling things via
kevent, or creating custom work queues (because kevent can deadlock on them).

This patch changes this to use a request_module_nowait() function macro instead,
which just fires the modprobe off but doesn't wait for it, and thus avoids the
original deadlock entirely.

On my laptop this already results in one less kernel thread running..

(Includes Jiri's patch to use enum umh_wait)

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (bool-ified)
Cc: Jiri Slaby <jirislaby@gmail.com>

include/linux/kmod.h

@@ -29,10 +29,15 @@
 #ifdef CONFIG_MODULES
 /* modprobe exit status on success, -ve on error.  Return value
  * usually useless though. */
-extern int request_module(const char * name, ...) __attribute__ ((format (printf, 1, 2)));
-#define try_then_request_module(x, mod...) ((x) ?: (request_module(mod), (x)))
+extern int __request_module(bool wait, const char *name, ...) \
+	__attribute__((format(printf, 2, 3)));
+#define request_module(mod...) __request_module(true, mod)
+#define request_module_nowait(mod...) __request_module(false, mod)
+#define try_then_request_module(x, mod...) \
+	((x) ?: (__request_module(false, mod), (x)))
 #else
 static inline int request_module(const char *name, ...) { return -ENOSYS; }
+static inline int request_module_nowait(const char *name, ...) { return -ENOSYS; }
 #define try_then_request_module(x, mod...) (x)
 #endif
 

kernel/kmod.c

@@ -50,7 +50,8 @@ static struct workqueue_struct *khelper_wq;
 char modprobe_path[KMOD_PATH_LEN] = "/sbin/modprobe";
 
 /**
- * request_module - try to load a kernel module
+ * __request_module - try to load a kernel module
+ * @wait: wait (or not) for the operation to complete
  * @fmt: printf style format string for the name of the module
  * @...: arguments as specified in the format string
  *
@@ -63,7 +64,7 @@ char modprobe_path[KMOD_PATH_LEN] = "/sbin/modprobe";
  * If module auto-loading support is disabled then this function
  * becomes a no-operation.
  */
-int request_module(const char *fmt, ...)
+int __request_module(bool wait, const char *fmt, ...)
 {
 	va_list args;
 	char module_name[MODULE_NAME_LEN];
@@ -108,11 +109,12 @@ int request_module(const char *fmt, ...)
 		return -ENOMEM;
 	}
 
-	ret = call_usermodehelper(modprobe_path, argv, envp, 1);
+	ret = call_usermodehelper(modprobe_path, argv, envp,
+			wait ? UMH_WAIT_PROC : UMH_WAIT_EXEC);
 	atomic_dec(&kmod_concurrent);
 	return ret;
 }
-EXPORT_SYMBOL(request_module);
+EXPORT_SYMBOL(__request_module);
 #endif /* CONFIG_MODULES */
 
 struct subprocess_info {