unlocked_ioctl vs normal ioctl

Question

In my driver's file_operations structure, I have:

struct file_operations Fops = {
  read:    device_read,
  write:   device_write,
  unlocked_ioctl:   device_ioctl,
  ...
};

I.e. there is no ioctl field used. Is this sufficient to avoid Big Kernel Lock and enter into device_ioctl() without any synchronization? Or do I have to change ioctl() calls in userspace part of the code too?

Answer 1

Uhm, I solved this. It is also required to change signature of device_ioctl function. There is no inode parameter, and also the function should return long. Just like in following patch:

-static int st_ioctl(struct inode *inode, struct file *file,
- unsigned int cmd_in, unsigned long arg)
+static long st_ioctl(struct file *file, unsigned int cmd_in, unsigned long arg)
{

(from: http://linux.derkeiler.com/Mailing-Lists/Kernel/2008-01/msg06799.html)

Answer 2

Read this LWN article: http://lwn.net/Articles/119652/

Also sometime between 2.6.33 and a 2.6.35 rc (use git-diff to find out which commit) the kernel now WARNs when only .ioctl is defined.

This is a move towards more explicit and fine-grained locking. Also note only changing the function signature and pointer will compile but will introduce the possibility of race conditions (two userspace apps doing ioctl calls at same time).