The difference among ioctl, unlocked_ioctl and compat_ioctl (RT)
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-runningioctl()
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.
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