On 17/10/2021 15:50:11+0200, Greg KH wrote:

Note, review of this now that it has been submitted in a pull request to
me, sorry I missed this previously...

On Wed, Sep 29, 2021 at 12:15:59PM +0900, William Breathitt Gray wrote:

quoted

+static int counter_chrdev_open(struct inode *inode, struct file *filp)
+{
+	struct counter_device *const counter = container_of(inode->i_cdev,
+							    typeof(*counter),
+							    chrdev);
+
+	/* Ensure chrdev is not opened more than 1 at a time */
+	if (!atomic_add_unless(&counter->chrdev_lock, 1, 1))
+		return -EBUSY;

I understand the feeling that you wish to stop userspace from doing
this, but really, it does not work.  Eventhough you are doing this
correctly (you should see all the other attempts at doing this), you are
not preventing userspace from having multiple processes access this
device node at the same time, so please, don't even attempt to stop this
from happening.

So you can drop the atomic "lock" you have here, it's not needed at all.

Could you elaborate a bit here because we've had a similar thing in the
RTC subsystem:

https://elixir.bootlin.com/linux/latest/source/drivers/rtc/dev.c#L28

And it would mean I can remove rtc->flags completely.


-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com