If you ever wondered how the Linux kernel is put together, here's an excellent description (complete with historical context) from Greg Kroah-Hartman, who knows it from the inside:
It used to be that Linus was the only one who could commit anything, or who would accept anything. It was all done through email.
And then BitKeeper was developed, and a number of us started using that. It was very nice, because we had a much faster feedback loop with Linus. I could have him pull 50 patches, see that he pulled them, and then I could hit him with another 50 patches, if I wanted to, within a day. Before, we had a lag of a couple of weeks.
Then, when we couldn’t use BitKeeper anymore, Linus wrote Git, and then that just increased it even more, because everybody can use Git. Our development model increased its capacity, and part of that was that we started trusting more people. Now we have subsystem maintainers and such.
As I said, I maintain the subsystems such as USB, and I have people who I trust enough that if they send me a patch, I’ll take it, no questions asked. Because the most important thing is I know that they will still be around in case there’s a problem with it. [laughs]
And then I send stuff off to Linus. So, Linus trusts 10 to 15 people, and I trust 10 to 15 people. And I’m one of the subsystem maintainers. So, it’s a big, giant web of trust helping this go on.
We’ve developed some procedures that help guide how and when we do merges and releases. And we’ve a very regular release schedule: every three months, we do a release. We have a two-week merge window, and all those patches in that merge window have to have been tested. We’ve gotten the development process down really well over the past four years.
We’re also increasing the rate of change in our development. The same amount of work one of the top 10 developers did last year wouldn’t have even made it into the top 20 this year. Our individual developers have got the work flow down, so we can actually contribute more, to an extent that’s amazing.
As you can see from this, kernel development is actually proceeding at a faster pace than in the past. Here are some more amazing facts about kernel development:
we add 11,000 lines, remove 5500 lines, and modify 2200 lines every single day.
People ask whether we can you keep that up, and I have to tell you that every single year, I say there’s no way we can go any faster than this. And then we do. We keep growing, and I don’t see that slowing down at all anywhere.
I mean, the giant server guys love us, the embedded guys love us, and there are entire processor families that only run Linux, so they rely on us. The fact that we’re out there everywhere in the world these days is actually pretty scary from an engineering standpoint. And even at that rate of change, we maintain a stable kernel.
Kroah-Hartman also makes this important – and really pretty extraordinary – point about the open source development of Linux:
It would actually be impossible at this point to create an operating system to compete against us. You can’t sustain that rate of change on your own, which makes it interesting to consider what might come after Linux.
For my part, I think the only thing that’s going to come after Linux is Linux itself, because we keep changing so much. I don’t see how companies can really compete with that.
Against that background, you almost begin to feel sorry for Microsoft...
This long interview with Kroah-Hartman is simply the best introduction to the inner workings of the kernel development team that I've read: highly recommended.