(copied almost as is from kernel-rules/rules/DRVRULES_en.trl)

SUMMARY¶

Once you blocked a driver using `try_module_get()` you should unblock it by `module_put()`.

DESCRIPTION¶

There is a possibility to block unloading of a driver (kernel module). You might want to use it if the driver is currently in use by OS objects or user processes. However, its uncautious usage can undermine stability of the system as a whole.

Note that such calls can be nested (they increase/decrease semaphore counter internally). Therefore, you should call exactly one proceure that unblocks the driver for each preceeding blocking one. Otherwise, the driver will not be available for unloading--without any serious reason to put such a restriction.

LINKS¶

[Sample bugfix](http://lkml.org/lkml/2010/1/12/246) of `drivers/mtd/mtd_blkdevs.c`

EXAMPLE¶

Sample error:

static int dev_open(struct inode*, struct file*)
{
try_module_get(THIS_MODULE);
...
//the absence of the module_put(THIS_MODULE) call
return 0;
}

Sample error in `drivers/mtd/mtd_blkdevs.c`:

static int blktrans_open(struct block_device *bdev, fmode_t mode)
{
struct mtd_blktrans_dev *dev = bdev->bd_disk->private_data;
struct mtd_blktrans_ops *tr = dev->tr;
int ret = -ENODEV;

if (!get_mtd_device(NULL, dev->mtd->index))
goto out;

if (!try_module_get(tr->owner))
goto out_tr;

/* FIXME: Locking. A hot pluggable device can go away
(del_mtd_device can be called for it) without its module
being unloaded. */
dev->mtd->usecount++;

ret = 0;
if (tr->open && (ret = tr->open(dev))) {
dev->mtd->usecount--;
put_mtd_device(dev->mtd);
out_tr:
module_put(tr->owner);
}
out:
return ret;
}

1. If `try_module_get(tr->owner) == 0` then we goto label `out_tr`;
2. At label `out_tr` we call `module_put` on the unloaded driver;

To fix the error we should replace:

goto out_tr;

with

goto out;