What Happens To -AC (And Other) Kernel Mods?

RedLeg wrote with this poser: "So, looking at the changelog for the 2.4.9 kernel release, I see a few '- Alan Cox: driver merges' entries. Intelligent consumers of (or those of us who modify them for our own uses) RedHat Kernel src.RPMs look at the patches in the RH kernel builds. Alan's (and other persistent RH) patches don't seem to be integrated into Linus' 'mainstream' kernel trees on any kind of a predictable basis, and this frequently causes projects like freeswan to have difficulty merging their patches (not intended for kernel inclusion) with kernels that appear 'in the wild' like the kernel RPMs from RedHat. Often, kernel patches for obviously older kernel versions continue to be applied (in the RPMs) to newer kernel versions. Alan is a RedHat-er, so he obviously has an inside track to RedHat kernel builds, but he's also Linus' Right-Hand man, but his patches are not (apparently) consistently making it into the 'mainstream' kernel. What am I missing?" Who better to answer this question than Alan Cox? Alan was kind enough to write an explanation of the (still complicated) process of merging -- and it's not as simple as who works for what distro maker ;)

Note: Here's what Alan passed on in response to this question. As usual, things aren't quite as simple as they first appear. -T.

Alan Cox: Probably the first thing to explain is the Red Hat kernel. That actually isn't something I am responsible for. Arjan van de Ven is the keeper of the distribution kernel, and has the unenviable task of getting a kernel together that will actually pass all the brutal QA testing. Arjan is perfectly entitled to (and sometimes does) throw out bits of -ac changes.

You'll see Red Hat patches being merged into -ac and Linus trees when appropriate, often from Arjan or Pete Zaitcev. Many of the other patches in the RH tree are considered "fixups" - they are workarounds for problems but not generalised or clean enough to feed into the main tree without further work. Others are RH specific patches for things like packaging.

With the -ac tree I try and do rapid rolling releases, sucking in new code to test it and also its interactions with other new code. By doing releases every few days I get a high number of people testing and reporting bugs before there are too many possible causes. This is how Linus trees used to work long ago, and I still think its the better technique.

At regular intervals I take stuff from the -ac tree and feed it to Linus. Sometimes Linus doesn't want to take other changes in case they confuse other things being done, sometimes they just vanish and fairly often they get applied.

I'm actually limited in the rate I can forward patches because I need to feed Linus blocks that are debuggable. Thus I don't want to feed Linus both file system and disk driver changes at once or I won't know which to blame if there are corruption reports.

I also don't feed Linus code that has active maintainers unless the maintainer has asked me to do so. Thus the USB diverges quite a lot because Johannes Erdfelt has chosen not to feed chunks of the USB and input changes on. Similarly, the user-mode-linux port in -ac has not been fed on to Linus because Jeff Dike wishes to improve it further before submitting it.

I have been concentrating on getting the driver code and some architectures synchronized with Linus, and that is now mostly done. The next big challenge is getting all the file system work on to Linus, and Al Viro has begun that and fed Linus the first blocks of the superblock handling cleanup.

Finally we have changes that are down to fundamental disagreements, perhaps in part stemming from the fact my background is real production systems rather than OS design work. Linus decided to update the 3D support without keeping back compatibility - I kept both. Linus I suspect will never accept a patch to do that. Secondly he decided that he didn't wish to allocate new device major numbers but look for a saner solution over time. Laudible, but not in the middle of a stable release. The -ac tree has drivers allocated "non-Linus" major numbers that are recognized by LANANA and thus common across vendors. These drivers like the HPT370 and Promise IDE raid will thus always be part of the -ac tree only.

The -ac tree also tries hard to avoid any incompatibilities. Having applications that require -ac or Linus trees is simply not an acceptable situation. The only specific exception for that right now for 2.4.x is deep at the system level and is for quota tools. That one was unavoidable to get 32bit uid quota working.

Comments

Post-Mortem debugging of multithreaded processes

[sllort]

One thing that's been in the -ac kernels for quite some time is the ability to post-mortem debug multithreaded processes. That is, under the production kernel, when you core dump, all the threading information is lost. You can't get the call stack of each thread. With the -ac kernels you got one core file per pid, with each LWP (lightweight process) getting its' own core file.

Considering that Solaris has had this (what seems to be BASIC) functionality for years, why do we see the continued insistence on keeping this functionality out of the production kernel? Are we waiting for the gdb team to catch up?

Until this is fixed, multithreaded programming under Linux will remain a black art - only developers willing to apply hordes of -ac patches to a homegrown development kernel have a change of successfully developing a multi-threaded application under Linux. Considering that many commercial software development packages (RogueWave, for instance) won't even support you if you're not using a RedHat released kernel, this puts multi-threaded development "out-of-bounds" for many.

Merge the -ac kernel mods!

Re:Yeah but

[Alan Cox]

The goal there is to make it unneccessary. 2.4.8-ac7/ac8 have slightly smarter VM merging behaviour done by Ben LaHaise for example.

I can appreciate the problem

[jd]

The FOLK project (gratuitous plug!) runs into all sorts of problems, all the time, from inconsistancies, patch for A being out of sync with patch for B, etc, etc, etc.

That's one of the reasons I started that project, in the first place. Because it's mind-numbingly tedious to massage patches from different groups together. If you can get the whole thing in one gigantic gloopy splodge, life would be much easier.

Unfortunately, I've discovered a number of things along the way:

• Debugging said gloopy splodge is a Royal Pain!
  
• Finding others who will help debug said gloopy splodge is not easy.
  
• Finding others who will even -report- bugs in said gloopy splodge isn't easy, ether.
  

That's not to say that FOLK is a disaster. Quite the opposite! I'm learning a huge amount about the Linux kernel, for a start, and the sheer complexity of juggling hundreds of patches is really giving my C coding skills a workout and a half!

My hat is off to Alan Cox who not only manages his patch set with far more grace than I ever could, but actually keeps it so that it runs!

I know the Royal Web Admin uses Linux (cos that was on an interview, some time ago), so if he's reading & has any influence, I honestly think Sir Cox would not be an undeserved title for his amazing computing skills and his contribution to both computing and Britain.

Re:I had no idea so much still needed work

[NNKK]

"Competing" ?
Without these alternate kernel trees, nothing would ever get done. the -ac trees really aren't a tree I'd recommend for a production server unless it has a fix or driver that the server desperately needs... the -ac's are to test and impliment things in advance to KEEP the "stable" tree "stable", and keep Linus happy that the patches he's putting in the mainstream tree have been tested.

And no, I don't feel the 2.4 series deserves to be called stable, but I damn well use it anyway on my primary desktop box :)

Linus, GIVE US 2.5 TO PLAY WITH!

Alan's Kernel is becoming more "official"

[trevorcor]

This is very interesting:
http://www.uwsg.indiana.edu/hypermail/linux/kernel /0108.2/0416.html

It seems Alan Cox is considering his -ac kernel tree to be a legitimate alternative to the official "Linus" tree, rather than a playground for testing patches. He's actually perpetuating a difference between -ac and the official tree, in a way that breaks source compatibility between the two (albeit in a very small way.) The fact that RedHat's kernels are all based on -ac now bears this out.

Alan has forked the Linux kernel.

::meyhem::

I think he has Linus' blessing in this though. Reading between the lines, I think Alan has been taking on more and more work in the past year or so that had previously been Linus'. Linux is ten years old now; I suppose Linus is burning out.

And Alan works for RedHat too, which is one of the two distributions that I *know* will be around in ten more years -- they have a solid business plan. Alan is voracious, just tireless, and RedHat would hire the entire core kernel development team if they had to. Linux will not die for lack of a maintainer if Linus gets hit by a bus tomorrow.