Kernel Traffic
Latest | Archives | People | Topics
Latest | Archives | People | Topics
Latest | Archives | People | Topics
Home | News | RSS Feeds | Mailing Lists | Authors Info | Mirrors | Stalled Traffic

Kernel Traffic #287 For 1 Jan 2005

By Zack Brown

Table Of Contents

Mailing List Stats For This Week

We looked at 2099 posts in 12934K.

There were 438 different contributors. 260 posted more than once. 162 posted last week too.

The top posters of the week were:

1. Linux 2.6.10-rc1-mm4 Released

9 Nov 2004 - 23 Nov 2004 (44 posts) Archive Link: "2.6.10-rc1-mm4"

Topics: Framebuffer, Kernel Release Announcement

People: Andrew Morton

Andrew Morton announced Linux 2.6.10-rc1-mm4, saying:

2. Out-Of-Memory Killer: Hunting For The Proper Layer

11 Nov 2004 - 24 Nov 2004 (57 posts) Archive Link: "[PATCH] fix spurious OOM kills"

Topics: Big Memory Support, OOM Killer

People: Marcelo TosattiAndrea ArcangeliAndrew Morton

Marcelo Tosatti said:

This is an improved version of OOM-kill-from-kswapd patch.

I believe triggering the OOM killer from task reclaim context is broken because the chances that it happens increases as the amount of tasks inside reclaim increases - and that approach ignores efforts being done by kswapd, who is the main entity responsible for freeing pages.

There have been a few problems pointed out by others (Andrea, Nick) on the last patch - this one solves them.

First, Andrea noted that if progress had been made in the high zone, the OOM killer would not be triggered. Now it conditions the triggering on "DMA+Normal" reclaiming success.

Nick noted that the last patch would do wrong on the NUMA case (because of per-node kswapd) - its not a problem now because we will only kill if there is any task not being able to allocate and free pages. The memory allocation fallback to other nodes will prevent that from happening.

Another drawback from the last patch was that it disable the "all_unreclaimable" logic which attempts to avoid scanning storms - that way kswapd was able to detect OOM condition.

What it does now is to disable the all_unreclaimable logic after 5 seconds that its been set. This is enough time for the system to complete IO of (at least some) pages which have been written-out for reclaiming purposes.

After that period (which can be sysctl'ed BTW), it then performs a full scan. In case no progress has been made, and both DMA and normal zones are below the pages_min watermark, the OOM killer is triggered.

It looks very reliable in my testing - but I need others to test it as well (Martin and Thomas especially who have good test cases).

Andrea Arcangeli felt it was wrong to have kswapd kill anything. He said, "kswapd is an async helper like pdflush and it has no knowledge on the caller (it cannot know if the caller is ok with the memory currently available in the freelists, before triggering the oom)." He went on to say that his own plan was to move the OOM killing code out of vmscan.c, which he said could not know which task was proper to kill, and into page_alloc.c, which could. He said, "vmscan.c can only know if something is still freeable, but if something isn't freeable it doesn't mean that we've to kill anything (for example if a task exited or some dma or normal-zone or highmem memory was released by another task while we were paging waiting for I/O). Every allocation is different and page_alloc.c is the only one who knows what has to be done for every single allocation."

Marcelo replied he had asked Andrea three times already whether Andrea had an idea of how exactly the proper method would work, and received no concrete answer each time. Andrea replied that his idea of moving the OOM killer into page_alloc.c was his proper method, but that he wouldn't have time to work on it for at least a week. Marcelo wished him luck. Apparently this means Andrea will get to complete his work before Marcelo decides on any final solution for 2.4. Elsewhere, Chris Ross tested Marcelo's original patch, but found that it would go haywire under certain circumstances.

Andrea said he'd get his patch out as soon as possible, but that in the mean time debugging Marcelo's patch would also help clarify other problems in that area of the kernel. The discussion continued, with Andrew Morton and others joining in; Andrea did not post his improvement during that discussion, however.

3. Linux 2.6.10-rc2 Released

14 Nov 2004 - 24 Nov 2004 (87 posts) Archive Link: "Linux 2.6.10-rc2"

Topics: Disks: IDE, FS: NTFS, Framebuffer, I2C, Kernel Release Announcement, PCI, Sound: ALSA, USB

People: Linus Torvalds

Linus Torvalds announced Linux 2.6.10-rc2, saying:

Ok, the -rc2 changes are almost as big as the -rc1 changes, and we should now calm down, so I do not want to see anything but bug-fixes until 2.6.10 is released. Otherwise we'll never get there.

A lot of driver updates, many of them of the small and trivial kind, others less so. USB, ALSA, fbdev, IDE, i2c, v4l, you name it. With a sprinking of core device model and PCI updates thrown in for good measure.

Also, a number of architecture updates: ppc64, m68k, uml, parisc, arm.

And md, NTFS, and Documentation updates too.

The diffstat shows more of the story - not so much single really big changes, more of a fairly wideranging thing.

4. Linux 2.6.10-rc2-mm1 Released

16 Nov 2004 - 22 Nov 2004 (23 posts) Archive Link: "2.6.10-rc2-mm1"

Topics: Kernel Release Announcement

People: Andrew MortonDave AirlieLee Revell

Andrew Morton announced Linux 2.6.10-rc2-mm1, with various fixes, but nothing major. Lee Revell noticed that VIA DRM had existed in 2.6.9-mm1, but was now gone. He asked why, and Andrew replied, "Actually I haven't been updating that for a while, because of ghastly conflicts upstream. Then it disappeared altogether due to administrative error. I'll see if I can resurrect it." Dave Airlie also explained, "I asked Andrew to kill it, I wasn't happy with it security wise still, resurrecting it could be messy as the tree isn't converted over to the core/library split, I'll probably pick it back up once Linus merges the current diffs after 2.6.10 is released... VIA DRM still only is useful for 2D HwMC stuff for non-root users, having to make a user run 3d apps as root is probably worse than having an in-secure DRM, so I'm still waiting for the VIA/unichrome people to see what they can do with it..." Lee replied, "FWIW almost no one uses the DRM for 3D on this card. Many people do use the 2D HwMC stuff though." And Dave said, "I might merge up a DRM with the 3D part disabled, so the 2D stuff works after 2.6.10 comes out, Alan wanted me to merge up all the outstanding drms that have security issues and disable them for non-root but someone pointed out the issues with making people run untrusted things as root to be able to use them at all..."

5. Linux 2.4.28 Released

16 Nov 2004 - 24 Nov 2004 (18 posts) Archive Link: "Linux 2.4.28-rc4"

Topics: Power Management: ACPI

People: Marcelo TosattiChris WrightBarry K. NathanOzkan Sezer

Marcelo Tosatti announced Linux 2.4.28-rc4, saying:

Here goes the fourth release candidate of 2.4.28.

A few small problems showed up in time for another -rc.

Missing exported symbols in the networking area, an ACPI poweroff bugfix, aic7xxx compile fix with -Werror, a binfmt_elf underflow enhancement, an SCTP fix, and a couple TG3 fixes.

This will become v2.4.28 final if nothing _really_ bad shows up.

Thanks to everybody who has been contributing to make this happen.

Ozkan Sezer pointed out some security fixes that had been recently posted; but Barry K. Nathan said he'd discussed them with Marcelo, and it was too late to include these in 2.4.28; Marcelo confirmed this, saying the patches would make it into 2.4.29-preX. Massimo Cetra asked why this decision had been made, if the security holes could be exploited. It seemed to him the most important thing was to close the holes before releasing 2.4.28 if possible. But Chris Wright pointed out that 2.4.28 had already been published by that time (Marcelo announced it as unchanged from -rc4); and Barry K. Nathan remarked that as far as he'd been able to determine, at the worst, an exploit would crash the program attempting to perpetrate the exploit - not such a terrible calamity.

6. Status Of OSS Deprecation

17 Nov 2004 - 18 Nov 2004 (9 posts) Archive Link: "[patch 2.6.10-rc2] oss: AC97 quirk facility"

Topics: Sound: ALSA, Sound: OSS, Sound: i810, Version Control

People: John W. LinvilleJeff GarzikJan EngelhardtLee RevellAlan CoxAndrew Morton

John W. Linville posted an OSS audio fix, "stolen shamelessly from ALSA" . Andrew Morton asked why not just use ALSA instead of OSS, and Jeff Garzik replied, "Until we actually remove the OSS drivers, it's sorta silly to leave them broken." Jan Engelhardt replied, "It's just as silly to fix something we're removing anyway." Alan Cox pointed out that a lot of folks still used OSS, and Jeff added, "Until it's gone, the current users would prefer not-broken to broken." Jan said it would be better to leave it broken, as an incentive to switch to ALSA; but Jeff said, "i810 audio still locks up in ALSA ATM..." At this point Lee Revell remarked, "Fixed in ALSA CVS on Tuesday. This fix needs to go in 2.6.10."

7. Xen 2.0 Updates

17 Nov 2004 - 18 Nov 2004 (9 posts) Archive Link: "Xen 2.0 VMM patches"

Topics: BSD: FreeBSD, BSD: NetBSD

People: Ian PrattAndi Kleen

Ian Pratt said:

The Xen team would like to start submitting upstream the patches required to enable Linux to run over the Xen 2.0 Virtual Machine Monitor. For more information about Xen see:

There are three main classes of patch required:

core : small patches that provide extra hooks for arch-xen
arch-xen : large patch to add arch/xen and include/asm-xen
xen-drivers : patch to add virtual block, network and console drivers

We're proposing to submit the 'core' patches first, as these probably require closest inspection (the others are all new files in new directories, so can't break existing architectures).

There is already a significant user base and lively developer community backing the Xen project, so we're confident we will have the resources to maintain arch-xen on Linux 2.6 going forward (as well as Linux 2.4/NetBSD/FreeBSD/etc).

Let us know what you think of the patches ;-)

Andi Kleen thought it didn't seem quite necessary to have a whole Xen port, paralleling the i386 tree. But Ian said:

We've experimented with doing thing the other way round, having xen a sub architecture of i386, but it was _really_ messy.

I firmly believe that having a separate arch/xen is the best approach for the moment. In the future, it might make sense to merge arch xen into i386, but to do this cleanly would require significant restructuring of i386. I think that's something we could move toward after everyone's gotten comfortable with having arch xen in the tree.

The fact that arch xen is self contained actually makes it easier for us to maintain in some respects. We've been tracking 2.6 releases for some time without too much difficulty.

Andi replied:

2.6 has been relatively easy for now (because it was supposed to be a "stable kernel"), but I suspect it'll get worse again over time. e.g. in 2.5 it was really bad for long times. Essentially you will need to commit significant man power to this.

Also it's quite hard to always catch all the changes that get done to i386.

Overall I think it's a bad idea to have four different x86 like architectures in the tree. Especially since there will be likely more hypervisors over time. i386 and x86-64 make some sense because 64bit is a natural boundary, but extending it elsewhere doesn't scale very well.

Ian said, "Over time, I hope we can merge the i386 xen port in with the native architecture, but its going to require significant restructuring of the native architecture to do it cleanly. In the meantime, I think we just have to bite the bullet and maintain a separate architecture. I believe we have resources to do this."

8. Linux 2.6.10-rc2-mm2 Released

18 Nov 2004 - 21 Nov 2004 (30 posts) Archive Link: "2.6.10-rc2-mm2"

Topics: Kernel Release Announcement

People: Andrew Morton

Andrew Morton announced Linux version 2.6.10-rc2-mm2, saying:

9. Kprobes Wrapper To Define jprobe.entry

18 Nov 2004 - 19 Nov 2004 (6 posts) Archive Link: "[PATCH] Kprobes: wrapper to define jprobe.entry"

Topics: Executable File Format

People: Ananth N. MavinakayanahalliAndrew MortonDavid S. Miller

Ananth N. Mavinakayanahalli said:

Here is a patch that adds a wrapper for defining jprobe.entry to make it easy to handle the three dword function descriptors defined by the PowerPC ELF ABI.

Current patch against 2.6.10-rc2-mm1 + kprobes patch for ppc64.

Changes for adding this wrapper for x86, ppc64 (tested) and x86_64 (untested) below. The earlier method of defining jprobe.entry will continue to work.

He posted some pseudocode to use jprobes with this patch, adding, "I am not aware of the semantics for sparc64 for making this change."

Andrew Morton said he hadn't seen the kprobes patch for ppc64 yet, and said, "what should I do with this? I'm inclined to drop it until the x86_64 part has been tested and Dave has had a go at the sparc64 version." David S. Miller remarked, "Yes, now that we have kprobe support on 4 platforms, it is important that anyone who changes public parts of this interface do the necessary per-platform fixups necessary to coincide with such changes. I think the person changing the data type should be the one fixing up sparc64 :-)" Ananth also replied to Andrew, saying, "I have now tested the patch succesfully on x86_64 and updated it for sparc64 too (Dave says the change looks good)." Andrew asked how much review and testing Ananth's kprobes-for-ppc64 patch had received so far, and Ananth replied, "The patch was earlier posted on the PPC64 mailing list for comments and Paul had reviewed it. I had to update the patch to take care of the base kprobe changes that were made to accomodate the x86_64 port. The patch is tested on POWER3 (uni) and POWER4 (lpar)."

10. LKST 2.2.0 For Linux 2.6.9 Released

18 Nov 2004 (1 post) Archive Link: "[ANNOUNCE/LKST] LKST 2.2.0 for linux-2.6.9 is released."

People: Masami Hiramatsu

Masami Hiramatsu said:

We are pleased to announce releasing new version of Linux Kernel State Tracer.

The Linux Kernel State Tracer(a.k.a. LKST) version 2.2.0 has been released. This version can be applied to linux-2.6.9. And LKST 2.2.0 supports IA32, IA64 and x86-64 platforms.

LKST is a tool that supports to analyze of fault and evaluate for kernel. Especially it is usuful for analyzing the unanticipated fault of kernal.

The latest version of the LKST newly supports the x86-64 architecture.

For more changes, see Changelog-2.2.0.txt

For supporting IA64 and x86-64, we hacked KernelHooks.

LKST binaries, source code and documents are available in the following site,

We prepared a mailing list written below in order to let users know update of LKST.

To subscribe, please refer following URL,

And if you have any comments, please send to the above list, or to another mailing-list written below.

11. Linux 2.6.9-ac10 Released

18 Nov 2004 - 19 Nov 2004 (4 posts) Archive Link: "Linux 2.6.9-ac10"

Topics: FS: smbfs, Forward Port

People: Alan CoxArjan van de Ven

Alan Cox, having resumed his fork of the kernel source, announced Linux 2.6.9-ac10, saying:

More merges and forward ports. These include the extra binfmt_elf checks and the SMBfs overflow fixes that match those in 2.4.28. It also includes several patches Dave Miller believes should be backported from the networking code. Also fixed is a long standing problem where ide would claim some devices it couldn't actually use and didn't install drivers for.

Arjan van de Ven is now building RPMS of the kernel and those can be found in the RPM subdirectory and should be yum-able. Expect the RPMS to lag the diff a little as the RPM builds and tests do take time.

The it8212 still doesn't default to DMA on - that is on the TODO list. The HPT366 rework project is also not ready (its gone back to the drawing board for a few days if you are a volunteer and wondered what is up).

12. Intel Thermal Monitor For x86_64 Updated

18 Nov 2004 - 19 Nov 2004 (6 posts) Archive Link: "[PATCH] Intel thermal monitor for x86_64 (updated)"

Topics: Version Control

People: Zwane MwaikamboAndi Kleen

Zwane Mwaikambo said:

This updated patch adds support for notification of overheating conditions on intel x86_64 processors and incorporates suggestions from Andi. Tested on EM64T and test booted on AMD64.

Hardware courtesy of Intel Corporation

Andrew please backout the current patch and apply this one (verified it applies against -bk), if you'd prefer an incremental let me know.

After some back-and-forth with Andi Kleen, Zwane posted a new patch, but with no additional description of what had changed.

13. Possible GPL Violation By Silicon Laboratories

18 Nov 2004 (2 posts) Archive Link: "Linux support for SiLabs CP2102 devices"

Topics: USB

People: Greg KH

Greg KH said:

I've been getting a lot of requests lately to see if Linux supports the USB to serial device from Silicon Laboratories called the CP2102 chip. It turns out that the company is claiming Linux support, yet they are only shipping a binary driver for Red Hat Linux 9.0.

In talking with the company, they insist that they will not release the source code to this module, and they claim that they are not infringing on any rights by not doing so. I claim that this is not true, as to write a usb to serial driver for Linux you have to use the drivers/usb/serial/usb-serial.h header file which is specifically licensed under the GPL v2. This file contains inline functions and structures that all usb-serial drivers need to use in order to work properly.

In short, there's no way you can write a Linux usb-serial driver, that uses the usbserial interface, without it being a derived work of other, GPL only code.

So, they are in violation, so what. Well, I can't do much about this (due to my employer's rules about suing companies). But I can do my best to spread the word that the CP2102 device is not supported on Linux, and should be avoided at all costs by anyone considering such a device in a future design. I encourage everyone else to help spread this information too.

If people are looking for a good usb to serial chip that is supported on Linux, Windows, and OS-X, there's the PL2303 device from Prolific, and the FTDI-SIO chip, and the MCT-U232 chip. All of these work very well on Linux, and are fully supported by all distros. I think they even might be cheaper than the CP2102 device too :)

Oh, and just for fun, attached to this message is the Linux driver that SiLabs is distributing, if anyone wants to poke around in it. The tarball contains 2 binary drivers, one of them a version of the usbserial.c file (which plainly is licensed under the GPL) and a mcci_usb.o binary driver. Have fun with it, but don't blame me for any badness that might happen to your box for running it, no one has any way of knowing exactly what this driver is doing.

So, in conclusion, please stay away from Silicon Laboratories devices, if you want to run Linux, as they are obviously not supporting Linux in any way.

There was no discussion of this, aside from Bill Marr thanking Greg for pointing out the violation.

14. New Smaller /sbin/hotplug Tool

18 Nov 2004 - 19 Nov 2004 (5 posts) Archive Link: "[ANNOUNCE] a very tiny /sbin/hotplug"

Topics: Hot-Plugging, Klibc, Small Systems

People: Greg KHEugene SuroveginRobert SchwebelChris Larson

Greg KH said:

a number of people have complained over the past few years about the fact that /sbin/hotplug was a shell script. Funny enough, it's the people on the huge boxes, with huge number of devices that are complaining, not the embedded people with limited resources (ironic, isn't it...)

Anyway, attached below is a replacement /sbin/hotplug written in c. Compiling it with klibc gives you me the following size:

$ size hotplug
   text    data     bss     dec     hex filename
   4149      28      20    4197    1065 hotplug
$ ls -l hotplug
-rwxr-xr-x  1 greg users 4636 Nov 18 15:08 hotplug

Which is smaller than /bin/true on my boxes (and /bin/true is linked dynamically, this is a static binary. Linked dynamically, it's still smaller than /bin/true. gnu programs, go figure...)

I'll be putting this all togther in a "hotplug-ng" type tarball, as I slowly replace the existing hotplug scripts with rewrites based on the fact that we've learned things over the past 4 years, and dropping support for 2.4 and previous kernels. But for now, have fun with this program.

Oh, and yeah, I know I need to fix up the fact that if /dev/null isn't present, we should create it ourselves and go from there. That's next on the list after putting it all in a project.

Remarking on why the complaints had come from big box users rather than embedded systems folks, Eugene Surovegin said, "This is probably because embedded people don't use hotplug at all :). On dozen different PPC and MIPS boxes I worked on, we never needed this feature." Robert Schwebel disagreed, saying, "PTXdist has hotplug support for some time." And Chris Larson also pointed out, "I tend to focus on ARM, and find this very useful. I know of a number of folks using erik andersen's "diethotplug" (which hasnt been touched in some time). I look forward to your further hotplug improvements." Greg replied:

Yes, diethotplug is based on my old package called, supprise, diethotplug, and is located at

But that only does module loading. The "full" replacement of today's /sbin/hotplug functionality needs this hotplug multiplexer, and a diethotplug replacement (which can get much smaller than the current diethotplug due to the way modprobe works in 2.6.)

15. Documenting ioctls

18 Nov 2004 - 19 Nov 2004 (6 posts) Archive Link: "IDE ioctl documentation & a new ioctl"

Topics: Disks: IDE, Ioctls

People: Edward FalkJim Nelson

Edward Falk said:

Hi all; let me introduce myself: I'm the guy that does IDE sustaining for Google.

I'm getting ready to sit down and document the IDE ioctls. Probably Documentation/hdio.txt or something like that. Before I start though, is anybody already doing this?

Jim Nelson said he didn't think anyone else was working on ioctl documentation. He said:

I had a thought in the back of my head of tackling ioctl documentation after I went through the stuff that's already in Documentation, but I figured I had enough to chew on for right now.

I'd probably make a subdirectory - i. e. Documentation/ioctl/hdio.txt - to differentiate it from other documents, and make it easier to get maintainers to put some stuff in there. ;) AFAICT, there is next to no documentation on ioctl's anywhere in the kernel tarball.

Edward liked the idea of an ioctl directory, as it would encourage folks to add to it.

16. Maintainership Of Orphaned Code

19 Nov 2004 - 20 Nov 2004 (5 posts) Archive Link: "[2.6 patch] remove obsolete Computone MAINTAINERS entry (fwd)"


People: Adrian BunkJim NelsonMichael H. WarfieldAndrew Morton

Adrian Bunk said, "I'm not sure whether it makes sense to list the previous maintainers for orphaned code, but if such entries contain buouncing mail addresses it's IMHO time to simply remove them." He posted a patch to remove Michael H. Warfield as the maintainer of the orphaned Computone Intelliport Multiport card. Andrew Morton said that the address listed in that Maintainer entry was still valid. Adrian replied that the mailing list was clearly defunct. Jim Nelson said:

If someone is willing to step up to the plate and get the driver working again, it would be better to have the contact information for the old maintainer available in a centralized location. Plus, anyone who tries the driver and finds it busted can look in the MAINTAINERS file and find out that it's not being fixed, instead of it being one of those drivers that has no defined maintainer (I've run across a few).

After all, we do have to be honest about unmaintained drivers - rather than hiding broken code. Honestly, it's better to define who's responsible for an area of code, even if it no longer works.

Adrian pointed out that the contact information was still available in the driver documentation; and that:

MAINTAINERS does not cover all drivers. It's not about being honest or about hiding broken code, but adding let's say fifty entries for no longer supported drivers isn't of any practical value.

At least I'm checking MAINTAINERS for people who want to receive the patches I send today, not for people who have worked on the driver at some time in the past. Adding information about orphaned drivers to this file simply makes it harder to find whether valid entries covering the driver in question exist.

If someone steps up and wants to generate and maintain a driver status document that would be fine with me. But this would require a serious amount of work and is more than just adding a few orphaned drivers to MAINTAINERS.

17. Some Discussion Of Compiler Extensions

20 Nov 2004 - 23 Nov 2004 (17 posts) Archive Link: "sparse segfaults"

People: Linus TorvaldsJan EngelhardtMitchell Blank JrRussell King

In the course of discussion, Russell King pointed out that the "int tickadj = 500/HZ ? : 1" C construction was pretty strange in kernel/ptrace.c. As someone else said, the 'true' part of the conditional was empty, and the C standard didn't specify a default in that case. Therefore it didn't make sense that a C compiler should be able to handle that construction. Linus Torvalds pointed out:

Actually, this is documented gcc behaviour, where a missing true condition is substituted with the condition value.

So what the above does is set "tickadj" to 500/HZ _except_ if that underflows to zero, in which case tickadj gets the value 1.

IOW, it's the same as

int tickadj = 500/HZ ? 500/HZ : 1;

except that the short syntax is not only shorter, but it's extremely convenient in macros etc, because it only evaluates the value once, ie you can do

int tickadj = *ptr++ ? : 1;

and it's well-behaved in that it increments the pointer only once.

Jan Engelhardt objected, "it's specific to GCC. This kinda ruins some tries to get ICC working on the kernel tree :)" Linus replied:

Ahh, but Intel compiler developers are cunning, and can make (and afaik, _did_ make) icc compatible with the good gcc extensions.

And as we all know, the definition of "good gcc extension" really is "the kernel uses it". (Some of the good ones turned up in C99 and are thus no longer extensions - obviously gcc wasn't the only compiler that implemented them).

Good gcc extensions:

BAD gcc extensions:

Jan Engelhardt took umbrage at Linus' categorization of bad GCC extensions, saying, "You don't have to use them..." But Linus replied:

We don't, generally. But they are bad even if you DON'T use them, because they sometimes make obvious syntax errors etc much harder to debug.

For example, the "nested function" thing makes something as simple as a missing end brace cause the error reporting to be totally off, when gcc decides that "hey, that's ok, those other function declarations are just nested functions in the middle of that other function". So you get something like

file.c: lastline: parse error at end of input

even though the _real_ parse error could have been pinpointed exactly if gcc did NOT do it's totally braindamaged nested functions. IOW, the extension causes problems even when you don't use it.

Same goes for the "extended lvalues". They are not only insane, but they mean that code like

(0,i) = 1;

actually compiles. Why is that a problem? Because some people (ie me) have used constructs like this in macros to make sure that the macro is "read-only", ie you have a situation where you don't want people to mis-use the macro on some architecture. So having

int max_of_something;
#define MAX_SOMETHING (0,max_of_something)

is actually a nice way to make sure nobody does anything like


but the gcc extension means that this doesn't actually work.

(Yes, I've been bitten by this. And no, I don't see the _point_ of the extension - does anybody actually admit to ever _using_ comma- expressions for assignments?)

Mitchell Blank Jr. said that when he wanted to do something similar to Linus' trick, he used "#define MAX_SOMETHING (max_of_something + 0)". He added, "When gcc accepts an arbitrary algebraic expression as an lvalue I'll be impressed :-)" Linus said:

Yes, I think I've done that too, and that works. My point is that the silly comma-expression should _also_ work, and I don't see any _valid_ use of the comma-expression as an lvalue.

I suspect (but don't have any real argument to back that up) is that the gcc "extended lvalues" fell out as a side effect from how gcc ended up doing some random semantic tree parsing, and it wasn't really "designed" per se, as much as just a random implementation detail. Then somebody noticed it, and said "cool" and documented it.

That's actually in my opinion a really good way to occasionally find a more generic form of something - the act of noticing that an algorithm just happens to give unintentional side effects that can actually be mis-used. So I don't think that it's a bad way per se to add features, especially as they then are often free (or even "negative cost", since _not_ adding the feature would entail having to add a check against it).

And for all I know, many of the _good_ gcc features ended up being done that way too.

But I think the (unintentional) downsides of these features are bigger than the advantages.

On further reflection, he added:

Btw, the "+0" thing is something that actually might be dropped pretty early, and as such a compiler _might_ get it wrong just because it ended up optimizing the expression away. So you don't need to be all that impressed, certain trivial expressions might just disappear under some circumstances.

Side note: the _biggest_ reason why "+0" is hard to optimize away early is actually type handling, not the expression itself. The C type rules means that "+0" isn't actually a no-op: it implies type expansion for small integer types etc.

So I agree that it's unlikely to be a problem in practice, but I literally think that the reason gcc ends up considering a comma-operator to be an lvalue, but not a +-operator really _is_ the type-casting issues. A comma doesn't do implicit type expansion.

What I find really strange is the ternary operator lvalue thing, though. A ternary operator _does_ do type expansion, so that extended lvalue thing is really quite complex for ternary ops. Try something like this:

        int test(int arg)
                char c;
                int i;

                return (arg ? c : i) = 1023;

and think about what a total disaster that is. Yes, gcc gets it right, but dammit, what a total crock. The people who thought of this feature should just be shot.

(Yes, it looks cool. Oh, well. The compiler can always simplify the expression "(a ? b : c) = d" into "tmp = d ; a ? b = tmp : c = tmp", but hey, so can the user, so what's the point? Looking at the output from gcc, it really looks like gcc actually handles it as a special case, rather than as the generic simplification. Scary. Scary. Scary.)

18. Hardware Troubles

24 Nov 2004 (1 post) Archive Link: " slowness"

People: H. Peter Anvin

H. Peter Anvin explained, "Just in case people have wondered about the recent slowness: we got unprecedented server load after Fedora Core 3 was released, and it hasn't fully died down yet. Additionally, the server is starting to show its age. We are actively working on getting new hardware; hopefully we should have something new in place some time in December or January."







Sharon And Joy

Kernel Traffic is grateful to be developed on a computer donated by Professor Greg Benson and Professor Allan Cruse in the Department of Computer Science at the University of San Francisco. This is the same department that invented FlashMob Computing. Kernel Traffic is hosted by the generous folks at All pages on this site are copyright their original authors, and distributed under the terms of the GNU General Public License version 2.0.