md: support disabling of create-on-open semantics.

md allows a new array device to be created by simply
opening a device file.  This make it difficult to
remove the device and udev is likely to open the device file
as part of processing the REMOVE event.

There is an alternate mechanism for creating arrays
by writing to the new_array module parameter.
When using tools that work with this parameter, it is
best to disable the old semantics.
This new module parameter allows that.
Signed-off-by: NeilBrown <neilb@suse.com>
Acted-by: Coly Li <colyli@suse.de>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -174,6 +174,16 @@ static const struct block_device_operations md_fops;
 
 static int start_readonly;
 
+/*
+ * The original mechanism for creating an md device is to create
+ * a device node in /dev and to open it.  This causes races with device-close.
+ * The preferred method is to write to the "new_array" module parameter.
+ * This can avoid races.
+ * Setting create_on_open to false disables the original mechanism
+ * so all the races disappear.
+ */
+static bool create_on_open = true;
+
 /* bio_clone_mddev
  * like bio_clone, but with a local bio set
  */
@@ -5285,7 +5295,8 @@ static int md_alloc(dev_t dev, char *name)
 
 static struct kobject *md_probe(dev_t dev, int *part, void *data)
 {
-	md_alloc(dev, NULL);
+	if (create_on_open)
+		md_alloc(dev, NULL);
 	return NULL;
 }
 
@@ -9200,6 +9211,7 @@ static int set_ro(const char *val, struct kernel_param *kp)
 module_param_call(start_ro, set_ro, get_ro, NULL, S_IRUSR|S_IWUSR);
 module_param(start_dirty_degraded, int, S_IRUGO|S_IWUSR);
 module_param_call(new_array, add_named_array, NULL, NULL, S_IWUSR);
+module_param(create_on_open, bool, S_IRUSR|S_IWUSR);
 
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("MD RAID framework");