What is the difference between ioctl(), unlocked_ioctl() and compat_ioctl()?

Question

Going through the linux 2.6.36 source code at lxr.linux.no, I could not find the ioctl() method in file_operations. Instead I found two new calls: unlocked_ioctl() and compat_ioctl(). What is the difference between ioctl(), unlocked_ioctl(), and compat_ioctl()?

Answer 1

Meta-answer: All the raw stuff happening to the Linux kernel goes through lkml (the Linux kernel mailing list). For explicative summaries, read or search lwn (Linux weekly news).

Answer: From The new way of ioctl() by Jonathan Corbet:

ioctl() is one of the remaining parts of the kernel which runs under the Big Kernel Lock (BKL). In the past, the usage of the BKL has made it possible for long-running ioctl() methods to create long latencies for unrelated processes.

Follows an explanation of the patch that introduced unlocked_ioctl and compat_ioctl into 2.6.11. The removal of the ioctl field happened a lot later, in 2.6.36.

Explanation: When ioctl was executed, it took the Big Kernel Lock (BKL), so nothing else could execute at the same time. This is very bad on a multiprocessor machine, so there was a big effort to get rid of the BKL. First, unlocked_ioctl was introduced. It lets each driver writer choose what lock to use instead. This can be difficult, so there was a period of transition during which old drivers still worked (using ioctl) but new drivers could use the improved interface (unlocked_ioctl). Eventually all drivers were converted and ioctl could be removed.

compat_ioctl is actually unrelated, even though it was added at the same time. Its purpose is to allow 32-bit userland programs to make ioctl calls on a 64-bit kernel. The meaning of the last argument to ioctl depends on the driver, so there is no way to do a driver-independent conversion.

Answer 2

There are cases when the replacement of (include/linux/fs.h) struct file_operations method ioctl() to compat_ioctl() in kernel 2.6.36 does not work (e.g. for some device drivers) and unlocked_ioctl() must be used.