unlocked_ioctl vs normal ioctl

c linux synchronization kernel ioctl
anon picture anon · Jun 30, 2009 · Viewed 24.3k times · Source

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

user318904 picture user318904 · Jul 12, 2010

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).

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