The final 2.6.24 Linux kernel is expected any day now, so the various subsystem maintainers have begun summarizing what changes are expected to be merged into the mainline kernel during the 2.6.25 merge window. Ingo Molnar spoke to changes for the x86 architecture, "there are 763 commits in x86.git so far, from more than 90 contributors, so it would be difficult to mention and credit every contribution in this mail." Along with a lengthy list of other changes, he included:

"Continued, intense arch/x86 unification and cleanup work by lots of people; FIFO ticket spinlocks for better spinlock scalability; 'regset' generalizations - the most important step towards utrace support (==next-gen ptrace); support for more than 255 CPUs [up to 4096 - in theory up to 65535]; almost complete 64-bit paravirt guest support; KGDB support on x86, finally!"

Linux creator Linus Torvalds announced the third release candidate for the upcoming 2.6.24 kernel summarizing, "hmmm.. Lots of small fixes, some cleanups, and a few things like the cris updates that aren't really either, but which won't affect any normal user, and will hopefully make it easier to sync up in the future. Network driver fixes, some IDE and infiniband updates, some late cpufreq updates, and a hwmon update." He continued:

"On the architecture side, in addition to the afore-mentioned cris updates, there are some sh, arm, powerpc and mips updates, and also one final x86 unification cleanup (and I really mean it - the rest can wait until after 2.6.24, but with this one the x86 configuration really is fairly merged, and both i386 and x86_64 are really just special cases of the 'x86' architecture in the configurator)."

"This series kill the old i386 and x86_64 directories. The relevant files are moved and adapted and Kconfig.debug was consolidated (thanks to Randy)," Sam Ravnborg said, describing a set of 6 patches to finish the migration of physical files into the new x86 architecture directory. He described it as "a nice patch series that deletes more lines than it adds," going on to explain:

"I had to modify both the top-level Makefile and the kconfig Makefile to accomplish this. It was done in such a way that it is trivial for other archs to use the same mechanism should they have the need.

"To solve the defconfig issue (i386 and x86_64 cannot share one) the arch/x86/configs/ directory were introduced. This has been used by other archs for some time now but x86 had not had the need until now."

"Can we please finish up this merge a little more before we freeze 2.6.24? The way we currently have leftovers of arch/i386/ and arch/x86_64/ is quite a nightmare and not how the other architectures were merged," Christoph Hellwig asked, leading to an insightful reply by Ingo Molnar. Ingo began by noting, "to answer that question one should first be aware of the fundamental code quality problems that the unified x86 architecture has inherited from the split i386 and x86_64 architectures." He then utilized the checkpatch script to generate a table of "coding style errors per one thousand lines of source code". In his table, arch/i386/ rated 77.3 errors per thousand lines of source, with arch/x86_64/ rating 96.0. The new unified arch/x86/ rated a lower but still very high 74.1. He summarized, "it is plainly obvious that the x86_64 and i386 architectures were in a dreadful state of code quality before the unification. Their code quality was almost an order of magnitude worse than that of the core kernel (!) - and their code quality was significantly worse than that of a couple of other, comparable architectures." Ingo continued:

"So to answer your question: full unification is no easy task and it is not automatic at all. The x86_64 tree has diverged from the i386 tree in the past 5 years due to their illogical, forced separation and a resulting bitrot. The two architectures have grown different sets of cleanliness problems and different sets of functions with arbitrary differences that often cover the same functionality. It's all compounded by the fact that the 64-bit code is in worse shape than the 32-bit - so it's not like we could just pick the 64-bit code and use that as the unified code. The 32-bit code is also used about 8-10 times more frequently than the 64-bit code. So there is no easy 'just unify it' path."

"This may count as one of the biggest -rc releases ever. It's humongous. Usually the compressed -rc1 diffs are in the 3-5MB range, with occasional smaller ones, and the occasional ones that top 6M, but this one is *eleven* megs," Linus Torvalds announced the first release candidate of the upcoming 2.6.24 kernel. He summarized some of the changes, "in short, we just had an unusually large amount of not just x86 merges, but also tons of new drivers (wireless networking stands out, but is by no means the only thing - we've got dvb, regular wired network, mmc etc all joining in), and a fair amount or architecture stuff, filesystems, networking etc too." He added:

"In other words, I don't even know where to start. The big noticeable thing is the x86 merge, and I think we all fervently hope that it won't cause any issues. So far it's been pretty smooth sailing. Knock wood. Less smooth has the scatter-gather changes to the block layer been, but they are hopefully all in reasonable shape by now too. And the VM changes? I honestly hope nobody even notices. Same goes for some of the VFS layer changes that affected basically every filesystem (although in mostly very straightforward ways)."

"Finally. Yeah, it got delayed, not because of any huge issues, but because of various bugfixes trickling in and causing me to reset my 'release clock' all the time. But it's out there now, and hopefully better for the wait," Linus Torvalds said announcing the 2.6.23 kernel. He noted few changes since the last release candidate, "not a whole lot of changes since -rc9, although there's a few updates to mips, sparc64 and blackfin in there. Ignoring those arch updates, there's basically a number of mostly one-liners (mostly in drivers, but there's some networking fixes and soem VFS/VM fixes there too)." Source level changes can be viewed via the gitweb interface. A nice overview of all changes can be found at Kernel Newbies. Linus went on to describe his plan going forward:

"I want this to be what people look at for a few days, but expect the x86 merge to go ahead after that. So far, all indications are still that it's going to be all smooth sailing, but hey, those indicators seem to always say that, and only after the fact do people notice any problems ;)"

"I said I was hoping that -rc8 was the last -rc, and I hate doing this, but we've had more changes since -rc8 than we had in -rc8. And while most of them are pretty trivial, I really couldn't face doing a 2.6.23 release and take the risk of some really stupid brown-paper-bag thing," Linus Torvalds said, announcing the 2.6.23-rc9 kernel. He added, "so there's a final -rc out there, and right now my plan is to make this series really short, and release 2.6.23 in a few days. So please do give it a last good testing, and holler about any issues you find!" He went on to warn developers that the first thing planned for 2.6.24 was to merge the unified x86 architecture:

"This is also a good time to warn about the fact that we're doing the x86 merge very soon (as in the next day or two) after 2.6.23 is out, so if you have pending patches for the next series that touch arch/i386 or x86-64, you should get in touch with Thomas Gleixner and Ingo Molnar, who are the keepers of the merge scripts, and will help you prepare..

"Doing it as early as possible in the 2.6.24-rc4 series (basically I'll do it first thing) will mean that we'll have the maximum amount of time to sort out any issues, and the thing is, Thomas and Ingo already have a tree ready to go, so people can check their work against that, and don't need to think that they have to do any fixups after it his *my* tree. It would be much better if everybody was just ready for it, and not taken by surprise."

Sam Ravnborg took a look at the x86 unification patches and commented, "from the mails and discussions I expected it be be obvious what was i386 only, what was shared and what was x86_64 only." He listed 16 files in x86/pci and noted, "in the filename there is NOTHING for most of the non-shared code that tell that this file is used by only i386 or x86_64." Andi Kleen concurred, "exactly my point from KS. The big mash-up will not really make much difference in terms of Makefile clarity or whatever Thomas' point was. Apparently he wanted to eliminate a few lines of code from the Makefile and merge the header files completely?"

Linus Torvalds disagreed saying, "the problem right now is the *reverse* - even though they are in different subdirectories (and thus *look* like they are all separate), they aren't. So the merged end result is much better: at a first approximation everything is shared (largely true), and the ones that aren't shared can be made more obvious." He added, "at least things like "grep" will work sanely, and people will be *aware* that 'Oh, this touches a file that may be used by the other word-size'." Linus continued:

"Right now, we have people changing 'i386-only' files that turn out to be used by x86-64 too - through very subtle Makefile things that the person who only looks into the i386 Makefile will never even *see*.

"THAT is the problem (well, at least part of it)."

Thomas Gleixner described an effort to create a unified x86 architecture tree, "the core idea behind our project is simple to describe: we introduce a new arch/x86/ and include/asm-x86/ file hierarchy that includes all the existing 32-bit and 64-bit x86 code and allows the building of either a 32-bit (i386) kernel or a 64-bit (x86_64) kernel." Andi Kleen expressed some concern, "I think it's a bad idea because it means we can never get rid of any old junk. IMNSHO arch/x86_64 is significantly cleaner and simpler in many ways than arch/i386 and I would like to preserve that. Also in general arch/x86_64 is much easier to hack than arch/i386 because it's easier to regression test and in general has to care about much less junk. And I don't know of any way to ever fix that for i386 besides splitting the old stuff off completely." Additional concerns about legacy issues were countered by Linus Torvalds, "there really isn't that much legacy crud. There are things like random quirks, but every time I hear the (theoretical) argument about how much time and effort we save by having it duplicated somewhere else, I think about all the time we definitely waste by fixing the same bug twice (and worry about the cases where we don't)." Among the justifications for a unified architecture, Thomas noted:

"We believe that the whole x86 CPU family is very much related and should be supported in a single architecture tree. All 64-bit CPUs implement the ability to execute pure 32-bit kernels, and will probably do so for the next couple of decades. So it's not like it will ever be possible to get rid of our legacies: for example even the latest 64-bit CPUs implement the legacy 'A20 line' feature that was already a weird outdated hack in the days of 16-bit x8086 CPUs."