lightnvm: prevent target type module removal when in use

If target type module e.g. pblk here is unloaded (rmmod) while module
is in use (after creating target) system crashes.  We fix this by
using module API refcnt.

Signed-off-by: Rakesh Pandit <rakesh@tuxera.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/lightnvm/core.c

@@ -22,6 +22,7 @@
 #include <linux/types.h>
 #include <linux/sem.h>
 #include <linux/bitmap.h>
+#include <linux/module.h>
 #include <linux/moduleparam.h>
 #include <linux/miscdevice.h>
 #include <linux/lightnvm.h>
@@ -316,6 +317,8 @@ static int nvm_create_tgt(struct nvm_dev *dev, struct nvm_ioctl_create *create)
 	list_add_tail(&t->list, &dev->targets);
 	mutex_unlock(&dev->mlock);
 
+	__module_get(tt->owner);
+
 	return 0;
 err_sysfs:
 	if (tt->exit)
@@ -351,6 +354,7 @@ static void __nvm_remove_target(struct nvm_target *t)
 
 	nvm_remove_tgt_dev(t->dev, 1);
 	put_disk(tdisk);
+	module_put(t->type->owner);
 
 	list_del(&t->list);
 	kfree(t);

drivers/lightnvm/pblk-init.c

@@ -1044,6 +1044,7 @@ static struct nvm_tgt_type tt_pblk = {
 
 	.sysfs_init	= pblk_sysfs_init,
 	.sysfs_exit	= pblk_sysfs_exit,
+	.owner		= THIS_MODULE,
 };
 
 static int __init pblk_module_init(void)