Kernel space: timerfd() and system call review

Fixing timerfd() might still be possible. But there is no denying that we would be better off if we could eliminate this kind of API problem before it gets into a stable kernel release and possibly has to be supported for many years. Therein lies the real problem: system calls (and other user-space API features) are being added to the kernel at a high rate, but review of these changes tends to lag behind. Given the difficulty of fixing user-space API mistakes, it would seem that the review standards for API additions should be especially high. Causing that to happen will not be easy, though; reviewer attention is a scarce resource throughout the free software community.

An idea which has been raised in the past is to explicitly mark new user-space interfaces as being in a volatile "beta" state. For as long as the API remains in that state, the kernel developers are free to change it. Applications would, during this period, rely in the API at their peril. This idea has been rejected in the past, though; it is seen as a way of avoid proper thought ahead of merging a new API into the kernel. Assuming that view still holds, another way will have to be found.

One part of the solution might well be seen in how the timerfd() problems came to light. Michael has demonstrated something the author has also encountered a number of times: one of the best ways to find shortcomings in an API is to attempt to document it comprehensively. If the kernel community were to resolve that it would not merge user-space API features in the absence of complete documentation, it might just provide the necessary incentive to get that last review pass done.

This idea seems likely to come up at next month's kernel summit (for which a preliminary agenda has just been posted). How it will be received is anybody's guess; writing documentation appears to be a task so challenging that even kernel hackers fear to try it. This challenge may be worth taking up, though, if the reward is few long-lasting user-space API problems in the future.