Understanding the Linux Kernel

4.8. Anticipating Linux 2.4

Linux 2.4 introduces a new mechanism called software interrupt. Software interrupts are similar to Linux 2.2's bottom halves, in that they allow you to defer the execution of a kernel function. However, while bottom halves were strictly serialized (because no two bottom halves can be executed at the same time even on different CPUs), software interrupts are not serialized in any way. It is quite possible that two CPUs run two instances of the same software interrupt at the same time. In this case, of course, the software interrupt must be reentrant. Networking, in particular, greatly benefits from software interrupts: it is much more efficient on multiprocessor systems because it uses two software interrupts in place of the old NET_BH bottom half.

Linux 2.4 introduces another mechanism similar to the bottom half called tasklet. Tasklets are built on top of software interrupts, but they are serialized with respect to themselves: two CPUs can execute two tasklets at the same time, but these tasklets must be different. Tasklets are much easier to write than generic software interrupts, because they need not be reentrant.

Bottom halves continue to exist in Linux 2.4, but they are now built on top of tasklets. As usual, no two bottom halves can execute at the same time, not even on two different CPUs of a multiprocessor system. Device driver developers are expected to update their old drivers and replace bottom halves with tasklets, because bottom halves ...