First, a little background for those of you who don’t live and breathe UNIX file systems performance. Ingo Molnar once called the access time, or “atime” feature of UNIX file systems “perhaps the most stupid Unix design idea of all times.” That’s harsh but fair. See, every time you read a file on a UNIX operating system – which includes OS X, Linux, and Android – it is supposed to update the file to record the last time it was read, or accessed. This is called the access time or atime. Cool, right? You can imagine why it’s helpful to know when was the last time anything read a particular file – you can tell if you have new mail, for example, or figure out which files you haven’t used in a while and can throw away.
The problem with the atime feature is that updating atime requires writing to the disk. So every read to a file creates a tiny disk write – and writes are expensive and slow. (SSDs don’t get rid of this problem; you still don’t want to do unnecessary writes and most of the world’s data is still on spinning disks.) Here’s what Ingo said about this in 2006: “Atime updates are by far the biggest IO performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_.“
So, atime is terrible idea – why don’t we just turn it off? That’s what many people did, using the “noatime” option that many file systems provide. The problem was that many programs did need to know the atime of a file to work properly. So most Linux distributions shipped with atime on, and it was up to the user to remember to turn it off (if they could). It was a bad situation.In 2006, I was a Linux file systems developer and also an active member of LinuxChix, a group for women who used Linux. LinuxChix existed in part because it was impossible to have technical discussions about Linux on most mailing lists without people insulting and flaming you for asking the simplest questions – and it was ten times worse for people with feminine usernames. Tell a cautionary story about installing RAM correctly, and the response might be a sneering, “Oh, you didn’t let out the magic smoke, did you?” On LinuxChix, that kind of obnoxiousness wasn’t allowed (though we still got a lot of what is now called mansplaining.)
So when I advised several people in LinuxChix to turn off atime, a friend felt safe telling me that hey, performance on her laptop was better, but now Mutt, the email reader we both used, thought she always had new email. This is because in her configuration, Mutt would look at an email file and compared its atime with the file’s last written time to figure out if any new email had arrived since the last time it read the file.
Now, the typical answer to “Mutt doesn’t work with noatime” was “Switch to a slower directory-based method,” or “Use a file size hack that had bugs,” or any number of other unhelpful things. Mostly, people just wouldn’t bother reporting things that broke with noatime. But I was part of a culture – a feminist culture – in which I respected people like my friend and programmers that attempted to use fully defined, useful features of UNIX in order to implement features efficiently.
I decided to look at the problem from a human point of view. What my friend and the Mutt programmers really wanted to know was this: Has this file been written since the last time I read it? They didn’t particularly care about the exact time of the last read, they just wanted to know if it had been read before or after the last write. I had an idea: What if we only updated a file’s atime if it would change the answer to the question, “Has this file been read since the last time it was written?” I called it “relative atime.”
The amazing thing is: it worked! Matthew Garrett (also a known feminist), Ingo Molnar, and Andrew Morton made some changes to patch, including updating the atime if the current atime was more than 24 hours ago. Other than that, this incredibly simple algorithm worked well enough that in 2009, relative atime became the default in the mainline Linux kernel tree. Now, by default, people’s computers were fast and their programs worked.
I came up with this idea and the original patch in 2006, when the atime problem had been known for many years. Previous solutions had taken a very file-system-centric point of view, mainly along the lines of buffering up atime updates in memory and writing them out when we ran out of memory. What led me to a creative, simple, and extremely fast solution was being part of a feminist community in which people felt comfortable sharing their technical problems, wanted to help each other, and respected each other’s intelligence. Those are all feminist principles, and they make file systems development better.
I try to take that human-centered, feminist approach with other topics in file systems, including the great fsync()/rename() debate of 2009 (a.k.a “O_PONIES”) in which I argued that file systems developers should strive to make life easier for developers and users, not harder. As recently as 2013, a leading file systems developer was still arguing that file systems didn’t have to save file data reliably by mocking users for playing computer games.
I was working on another human-centered file system feature, union mounts, when I heard that a friend of mine had been groped at an open source conference for the third time in one year. While I loved my file systems work, I felt like stopping sexual harassment and assault of women in open source was more urgent, and that I was uniquely qualified to work on it. (I myself had been groped by another Linux storage developer.) So I quit my job as a Linux kernel developer and co-founded the Ada Initiative, whose mission is supporting women in open technology and culture. Unfortunately, as a result of my work, several more Linux storage developers came out publicly in favor of harassment and assault.
That’s one reason why I’m so excited that Ceph developer Sage Weil challenged the open storage community to raise $8192 for the Ada Initiative by Wednesday, Oct. 8 – and he’ll personally match that amount if we reach the goal! UPDATED: Sage and Mike Perez raised this to $16,384!!! The number of Linux file systems and storage developers who both donated to Sage’s challenge and wanted to be listed publicly as supporters is reminding me that the vast majority of the people I worked with in Linux want women to feel safe and comfortable in their community. I love file systems development, I love writing kernel code, and I miss working with and seeing my Linux friends. And as you can tell by the lack of something like union mounts in the mainline kernel 21 years after the first implementation, Linux file systems and storage does not have enough developers, and can’t afford to keep driving off women developers.The Ada Initiative is capable of changing this situation. In August 2014, I taught the first Ally Skills Workshop at a Linux Foundation-run conference, LinuxCon North America. The Ally Skills Workshop teaches men simple everyday ways to support women in their workplaces in communities, and teaching it is my favorite part of my work. I was happy to see several Linux file systems and storage developers at the workshop. I was still nervous about running into the developers who support harassment and assault, but seeing how excited people were after the Ally Skills Workshop made it all worthwhile.
If you’d like to see more people working on Linux storage and file systems, and especially more women, please join Sage Weil and more than 30 other open storage developers in supporting the Ada Initiative. Donate now:
Edited to add 10/6/2014: Sage made his goal, hurray! And here’s my favorite comment from the HN thread about this story, the only one actually flagged into non-existence (plenty of other creepy misogyny elsewhere though):
Also, I had no idea Lennart Poettering planned to post this detailed description of the abuse, harassment, and death threats he’s suffered as an open source developer.
We’re still raising money for Ada Initiative to fight this kind of harassment, so feel free to donate:
 Yes, Android is Linux too, I’m just naming the brands that non-operating systems experts would recognize.
 “Relative atime” isn’t so bad, but the name of the option that you pass to the kernel, “relatime”, showed my usual infelicity with naming things as it looks like a misspelling of “realtime”.
Tagged: ada initiative, feminism, filesystems, kernel, linux
Aside from various minor changes to many pages, the most notable changes in man-pages-3.74 the following:
- A new cp1252(7) page by Marko Myllynen documents the CP 1252 character set. Marko also submitted a large number of patches that made the various character set pages in Section 7 more consistent content and layout.
- I've written a new pldd(1) page that documents the pldd command added to glibc in version 2.15.
- I've made a number of updates to the mq_overview(7) page, among them to add documentation of the /proc/sys/fs/mqueue/msgsize_default and /proc/sys/fs/mqueue/msg_default files.
On the modeset side of things we now have the final pieces for plane rotation support from Sonika Jindal and Ville. The DisplayPort code has also seen lots of improvements, with updated training values in preparation of the latest eDP standard (Sonika Jindal) and support for DP training pattern 3 (Ville). DSI panels now support burst mode (Shobhit) and hdmi conformance has been improved with some fixes from Clint Taylor.
For eDP panels we also have improved panel power sequencing code, mostly to fix issues on Cherryview, from Ville. Ville has also contributed fixes to the VDD handling code, which is used to temporarily enable panel power. And the backlight code learned to handle the bl_power setting so that the backlight can be turned off completely without upsetting the panel's power sequencing, contributed by Jani.
Chris Wilson has also been fairly busy on the modeset code: 3.18 includes his patches to cache EDIDs for a single probe call - unfortunately the full caching solution to keep the EDID around between multiple probe calls isn't merged yet. And pageflips have now improved error detection and recovery logic: In case something goes wrong we shouldn't end up stuck any longer waiting for a pageflip to complete that has been lost by either the hardware or the driver.
Moving on to platform specific work there's been lots of preparations for Skylake, most of it from Damien and Sonika. The actual intial platform enabling is delayed for 3.19 though. On the other end of the timeline Ville fixed up i830M modeset support on a rainy w/e in his vacation, and 3.18 now has all that code. And there has been a lot of Cherryview fixes all over.
Cherryview also gained support for power wells and hence runtime pm (Ville). And for platform agnostic feature a lot of the preparation for DRRS (dynamic refresh rate switching) is merged, hopefully the actual feature patches from Vandana Kannan will land in 3.19.
Moving on the render side of the driver there's been a lot of patches to beat the full ppgtt support into shape. The context code has been cleaned up, lifetime handling for ppgtt address spaces is fixed and bad interactions with secure batches are now also rectified. Enabling full ppgtt missed the feature cutoff by a hair though, but it's already enabling for the following release.
Basic support for execlists command submission from Ben Widawsky, Oscar Mateo and Thomas Daniel was also merged. This is the fancy new way to submit commands available on Gen8 and subsequent platforms. It's not yet enabled by default, but since it's a requirement for a lot of cool new features keep an eye on what's going on here. There is also a lot of work going on underneath to enable all this new code in GEM, like preparing to switch away from sequence numbers to tracking gpu progress more abstractly using the driver's request structures.
And this time around there is also some cool stuff going on in the drm core worth of a shout-out: The vblank handling code is massively revamped, hopefully plugging all the small races, inconsistencies and inefficiencies in that code. And thanks to David Herrmann it is finally possible to write a drm driver without the drm midlayer getting completely in the way of a proper driver load and unload sequence! Unfortunately i915 can't be converted right away since the legacy usermodesetting code crucial relies on this midlayer functionality. But that's now well deprecated and hopefully can be removed in one of the next releases.
Recently, as part of the anti-women #GamerGate campaign, a set of awful humans convinced Intel to terminate an advertising campaign because the site hosting the campaign had dared to suggest that the sexism present throughout the gaming industry might be a problem. Despite being awful humans, it is absolutely their right to request that a company choose to spend its money in a different way. And despite it being a dreadful decision, Intel is obviously entitled to spend their money as they wish. But I'm also free to spend my unpaid spare time as I wish, and I no longer wish to spend it doing unpaid work to enable an abhorrently-behaving company to sell more hardware. I won't be working on any Intel-specific bugs. I won't be reverse engineering any Intel-based features. If the backlight on your laptop with an Intel GPU doesn't work, the number of fucks I'll be giving will fail to register on even the most sensitive measuring device.
On the plus side, this is probably going to significantly reduce my gin consumption.
 In the spirit of full disclosure: in some cases this has resulted in me being sent laptops in order to figure stuff out, and I was not always asked to return those laptops. My current laptop was purchased by me.
 I appreciate that there are some people involved in this campaign who earnestly believe that they are working to improve the state of professional ethics in games media. That is a worthy goal! But you're allying yourself to a cause that disproportionately attacks women while ignoring almost every other conflict of interest in the industry. If this is what you care about, find a new way to do it - and perhaps deal with the rather more obvious cases involving giant corporations, rather than obsessing over indie developers.
For avoidance of doubt, any comments arguing this point will be replaced with the phrase "Fart fart fart".
 Except for the purposes of finding entertaining security bugs
I took a few months off from dealing with my spam problems, choosing to stick my head in the sand. Probably not my wisest move…
In the interim, the opendmarc developers have been busy, releasing version 1.3.0, which also adds the nice feature of doing SPF checking internally. This lets me CLOSE WONTFIX the smf-spf and libspf2 packages from the Fedora review process and remove them from my system. “All code has bugs. Unmaintained code with bugs that you aren’t running can’t harm you.” New packages and the open Fedora review are available.
I’ve also had several complaints from friends, all @yahoo.com users, who have been sending mail to myself and family @domsch.com. In most cases, @domsch.com simply forwards the emails on to yet other mail provider – it’s providing a mail forwarding service for a vanity domain name. However, now that Yahoo and AOL are publishing DMARC p=reject rules, after smtp.domsch.com forwarded the mail on to its ultimate home, those downstream servers were rejecting the messages (presumably on SPF grounds – smtp.domsch.com isn’t a valid mail server for @yahoo.com).
My solution to this is a bit akward, but will work for a while. Instead of forwarding mail from domains with DMARC p=reject or p=quarantine, I now store them and serve them up via POP3/IMAP to their ultimate destination. I’m using procmail to do the forwarding:
SENDER=`formail -c -x Return-Path`
SENDMAILFLAGS="-oi -f $SENDER"
# forward all mail except dmarc policy reject|quarantine.
* ? formail -x'From:' | grep -o '[[:alnum:]+\.\_\-]*@[[:alnum:]+\.\_\-]*' | xargs opendmarc-check | egrep -s 'Domain policy: (reject|quarantine)'
This introduces quite a bit of latency (on the order of an hour) for mail delivery from my friends with @yahoo.com addresses, but it keeps them from getting rejected due to their email provider’s lousy choice of policy.
Tim Draegen, the guy behind the excellent dmarcian.com, is chairing a new IETF working group focusing on proper handling on “indirect email flows” such as mailing lists and vanity domain forwarding. I’m hoping to have time to get involved there. If you care, follow along on their mailing lists.
I’m choosing to ignore the fact that domsch.com is getting spoofed 800k times a week (as reported by 8 mail providers and visualized nicely on dmarcian.com), at least for now. I’m hoping the new working group can come up with a method to help solve this.
Do your friends use a mail service publishing DMARC p=reject? Has it caused problems for you? Let me know in the comments below.
The Ada Initiative supports women in open technology and culture – looking around my place of work and various conference halls I’ve visited, I think there’s little doubt that we’ve still got an old boy network running here, and I’d like to see that change. I have daughters who may or may not get deep into tech culture, and I’m glad there are people like Val working to make it a better place for them if they do.
And she’s recently gotten a nice bit of potential help: Sage Weil of Ceph & Dreamhost fame has issued a challenge: Raise $8192 in 8 days, and he’ll match it. They’re already on their way. If you can help, please do. Power-of-two donations encouraged, but not required. :) Click the counter below to see their donation page. Thanks!
Here’s the intro to my first post, “Who am I?”:
The existential question, asked by everyone and everything throughout their lifetimes – who am I? High school seniors choosing a college, college seniors considering grad school or entering the job market, adults in the midst of their mid-life crisis—the question comes far easier than the answer.
In the world of technology, who you are depends on the technology with which you are interacting. On Facebook, you are your quirky personal self, with pictures of your family and vacations you take. On LinkedIn, you are your professional self, sharing articles and achievements that are aligned with your career.
What about on the myriad devices you carry around? On the smartphone in my pocket, I have several personas—personal, business, gamer (my kids borrow my phone), constantly context-switching between them. In the not-too-distant past, people would carry two phones—one for personal use and one for work, keeping the personas separate via physical separation—two personas, two devices.
I gave this talk at LPC 2012. It promotes the idea that programs layered on top of human-centric interfaces is a bad idea.
The timing of this post with the announcement of the most recent bash vulnerability is not entirely coincidental.
The authors of “How Google Works” have given electronic versions of “How Google Works” to all Google employees. Since I had already purchased a copy via pre-order, to make life interesting, I’ve decided to give my Google Play coupon code to someone via an electronic lottery. (Edit: the coupon code will only work for people with Google accounts in the US; if you live outside of the US, my apologies, but the coupon code will not work for you.)
I will be using the procedure documented by RFC-3797 to select someone from the list of people who have sent-email to email@example.com, snapshotted at Noon US/Eastern on Friday, September 26th, 2014, using as inputs into the RFC-3797 algorithm: (1) the daily volume of GOOG on September 26th, 2014 as reported by finance.google.com, (2) the daily volume of GOOGL on September 26th, 2014 as reported by finance.google.com, and (3) the Massachusetts Powerball Lottery Numbers for September 27th, 2014. If any of these values are not available for any reason, the values for the next trading day or lottery draw will be used.
The problem is that most people don't fall into either of these categories, and so the benefits of free software are often largely theoretical to them. Concentrating on philosophical freedoms without considering whether these freedoms provide meaningful benefits to most users risks these freedoms being perceived as abstract ideals, divorced from the real world - nice to have, but fundamentally not important. How can we tie these freedoms to issues that affect users on a daily basis?
In the past the answer would probably have been along the lines of "Free software inherently respects users", but reality has pretty clearly disproven that. Unity is free software that is fundamentally designed to tie the user into services that provide financial benefit to Canonical, with user privacy as a secondary concern. Despite Android largely being free software, many users are left with phones that no longer receive security updates. Textsecure is free software but the author requests that builds not be uploaded to third party app stores because there's no meaningful way for users to verify that the code has not been modified - and there's a direct incentive for hostile actors to modify the software in order to circumvent the security of messages sent via it.
We're left in an awkward situation. Free software is fundamental to providing user privacy. The ability for third parties to continue providing security updates is vital for ensuring user safety. But in the real world, we are failing to make this argument - the freedoms we provide are largely theoretical for most users. The nominal security and privacy benefits we provide frequently don't make it to the real world. If users do wish to take advantage of the four freedoms, they frequently do so at a potential cost of security and privacy. Our focus on the four freedoms may be coming at a cost to the pragmatic freedoms that our users desire - the freedom to be free of surveillance (be that government or corporate), the freedom to receive security updates without having to purchase new hardware on a regular basis, the freedom to choose to run free software without having to give up basic safety features.
That's why projects like the GNOME safety and privacy team are so important. This is an example of tying the four freedoms to real-world user benefits, demonstrating that free software can be written and managed in such a way that it actually makes life better for the average user. Designing code so that users are fundamentally in control of any privacy tradeoffs they make is critical to empowering users to make informed decisions. Committing to meaningful audits of all network transmissions to ensure they don't leak personal data is vital in demonstrating that developers fundamentally respect the rights of those users. Working on designing security measures that make it difficult for a user to be tricked into handing over access to private data is going to be a necessary precaution against hostile actors, and getting it wrong is going to ruin lives.
The four freedoms are only meaningful if they result in real-world benefits to the entire population, not a privileged minority. If your approach to releasing free software is merely to ensure that it has an approved license and throw it over the wall, you're doing it wrong. We need to design software from the ground up in such a way that those freedoms provide immediate and real benefits to our users. Anything else is a failure.
(title courtesy of My Feminism will be Intersectional or it will be Bullshit by Flavia Dzodan. While I'm less angry, I'm solidly convinced that free software that does nothing to respect or empower users is an absolute waste of time)
 Either in the sense of having enough money that you can simply pay, having enough background in the field that you can file meaningful bug reports or having enough followers on Twitter that simply complaining about something results in people fixing it for you
 The free software nature of Android often makes it possible for users to receive security updates from a third party, but this is not always the case. Free software makes this kind of support more likely, but it is in no way guaranteed.
Before everyone freaks out about Matthew Garrett’s post regarding ACPI on ARM, here are a few things to keep in mind:
First, when we’re talking about Linux and ACPI on ARM, we’re talking about general purpose servers. In the general purpose server market, Linux is already the dominant OS, regardless of the CPU architecture. Servers are designed, built and sold to run Linux. It is already the situation that x86 server vendors build their ACPI tables to work with Linux. Supporting Linux on ARM servers is merely an extension of what vendors are already doing to support Linux on x86. Despite Matthew’s concern, I don’t think we’re entering new territory in this regard.
Second, many of us have bad memories of getting ACPI to work with Linux. However, it is worth remembering that most of our problems have been with machines where the vendor really doesn’t care about Linux – usually desktop or laptop PCs. It’s not surprising that we have problems with these machines since they’ve only been tested with Windows! Server vendors, on the other hand, have a vested interest in ensuring that Linux runs well on their hardware and so they regularly test with Linux. The negative lessons learned in the laptop and desktop markets don’t carry over to machines built to run Linux.
Third, the ACPI world has changed in the last 2 years. It used to be that the ACPI spec was governed in a closed process by 5 companies: HP, Intel, Microsoft, Phoenix, and Toshiba, with nary a Linux person to be seen. Last year ACPI governance was transferred to the UEFI Forum and we’ve got plenty of Linux engineers sitting at the table. In light of that, it is no longer true that ACPI only caters to the needs of Windows, and we have the ability to propose changes to the spec. In fact, if you look at the revision history in version 5.1 of the spec, you’ll find changes that were proposed by Linux engineers to make ARMv8 work.
That said, the issues raised by Matthew are important. There is a big question about how Linux should declare itself to the platform. Claiming to be compatible with “Windows 8″ in the ACPI _OSI (Operating System Interface) method obviously isn’t appropriate on ARM. There is some talk about removing _OSI entirely on ARM since the way Linux uses it isn’t actually useful, and the _OSC (Operating System Capability) method has been proposed as a better way to declare what the OS supports. There is also a need to make sure vendors are testing with linux-next and mainline kernels so that we know when breakage happens and we can either do something about it, or work with vendors to fix their firmware.
Both of these are important issues and I think we need to propose solutions before merging ARM ACPI support into the kernel. Some of this work has already started: Linaro is running Canonical’s Firmware Test Suite (FWTS), the ACPI API tests, and the ACPI ASL tests on ARM, and we’re porting the Linux UEFI Verification (LUV) project which packages all the test suites into an easy to use distribution.
While I agree with Matthew that getting the interface between firmware and the OS is hard, I do not see the nightmare scenario he is describing. It certainly hasn’t played out that way on x86 servers where Linux is already the preferred OS. Besides, I really cannot agree with the premise that Linux being the dominant OS is a bad thing! We have a lot more influence than we give ourselves credit for.
The most notable changes in man-pages-3.73 are various new and modified pages describing namespaces in general, and user and PID namespaces in detail:
- A new namespaces(7) page provides an overview of Linux namespaces. Linux currently supports six namespaces (IPC, mount, network, PID, UTS, and user), and each namespace wraps some global system resource(s) in an abstraction that makes it appear to the processes within the namespace that they have their own isolated instance of the resource(s). Much of the content this and the other new pages in this release was drawn from a series of articles that I wrote for LWN.net in January 2013, but a few details have changed since and are documented the new man pages.
- A new pid_namespaces(7) page documents PID namespaces in detail.
- A new user_namespaces(7) page documents user namespaces in great detail.
- The clone(2) page adds documentation of the CLONE_USER flag, which is used for creating a child process in a new user namespace.
- The setns(2) page adds documentation of the CLONE_NEWNS, CLONE_NEWPID, and CLONE_NEWUSER flags.
- The unshare(2) page adds documentation of the CLONE_NEWPID, CLONE_NEWUSER, CLONE_SIGHAND, CLONE_THREAD, and CLONE_VM flags.
Inevitably, though, it can't define the full behaviour of an ACPI system. It doesn't explicitly state what should happen if you violate the spec, for instance. Obviously, in a just and fair world, no systems would violate the spec. But in the grim meathook future that we actually inhabit, systems do. We lack the technology to go back in time and retroactively prevent this, and so we're forced to deal with making these systems work.
This ends up being a pain in the neck in the x86 world, but it could be much worse. Way back in 2008 I wrote something about why the Linux kernel reports itself to firmware as "Windows" but refuses to identify itself as Linux. The short version is that "Linux" doesn't actually identify the behaviour of the kernel in a meaningful way. "Linux" doesn't tell you whether the kernel can deal with buffers being passed when the spec says it should be a package. "Linux" doesn't tell you whether the OS knows how to deal with an HPET. "Linux" doesn't tell you whether the OS can reinitialise graphics hardware.
Back then I was writing from the perspective of the firmware changing its behaviour in response to the OS, but it turns out that it's also relevant from the perspective of the OS changing its behaviour in response to the firmware. Windows 8 handles backlights differently to older versions. Firmware that's intended to support Windows 8 may expect this behaviour. If the OS tells the firmware that it's compatible with Windows 8, the OS has to behave compatibly with Windows 8.
In essence, if the firmware asks for Windows 8 support and the OS says yes, the OS is forming a contract with the firmware that it will behave in a specific way. If Windows 8 allows certain spec violations, the OS must permit those violations. If Windows 8 makes certain ACPI calls in a certain order, the OS must make those calls in the same order. Any firmware bug that is triggered by the OS not behaving identically to Windows 8 must be dealt with by modifying the OS to behave like Windows 8.
This sounds horrifying, but it's actually important. The existence of well-defined OS behaviours means that the industry has something to target. Vendors test their hardware against Windows, and because Windows has consistent behaviour within a version the vendors know that their machines won't suddenly stop working after an update. Linux benefits from this because we know that we can make hardware work as long as we're compatible with the Windows behaviour.
That's fine for x86. But remember when I said it could be worse? What if there were a platform that Microsoft weren't targeting? A platform where Linux was the dominant OS? A platform where vendors all test their hardware against Linux and expect it to have a consistent ACPI implementation?
Our even grimmer meathook future welcomes ARM to the ACPI world.
Software development is hard, and firmware development is software development with worse compilers. Firmware is inevitably going to rely on undefined behaviour. It's going to make assumptions about ordering. It's going to mishandle some cases. And it's the operating system's job to handle that. On x86 we know that systems are tested against Windows, and so we simply implement that behaviour. On ARM, we don't have that convenient reference. We are the reference. And that means that systems will end up accidentally depending on Linux-specific behaviour. Which means that if we ever change that behaviour, those systems will break.
So far we've resisted calls for Linux to provide a contract to the firmware in the way that Windows does, simply because there's been no need to - we can just implement the same contract as Windows. How are we going to manage this on ARM? The worst case scenario is that a system is tested against, say, Linux 3.19 and works fine. We make a change in 3.21 that breaks this system, but nobody notices at the time. Another system is tested against 3.21 and works fine. A few months later somebody finally notices that 3.21 broke their system and the change gets reverted, but oh no! Reverting it breaks the other system. What do we do now? The systems aren't telling us which behaviour they expect, so we're left with the prospect of adding machine-specific quirks. This isn't scalable.
Supporting ACPI on ARM means developing a sense of discipline around ACPI development that we simply haven't had so far. If we want to avoid breaking systems we have two options:
1) Commit to never modifying the ACPI behaviour of Linux.
2) Exposing an interface that indicates which well-defined ACPI behaviour a specific kernel implements, and bumping that whenever an incompatible change is made. Backward compatibility paths will be required if firmware only supports an older interface.
(1) is unlikely to be practical, but (2) isn't a great deal easier. Somebody is going to need to take responsibility for tracking ACPI behaviour and incrementing the exported interface whenever it changes, and we need to know who that's going to be before any of these systems start shipping. The alternative is a sea of ARM devices that only run specific kernel versions, which is exactly the scenario that ACPI was supposed to be fixing.
 Defined by implementation, not defined by specification
 Windows may change behaviour between versions, but always adds a new _OSI string when it does so. It can then modify its behaviour depending on whether the firmware knows about later versions of Windows.
I primarily work on Linux, so I put this in my Emacs config:
; Linux mode for C (setq c-default-style '((c-mode . "linux") (other . "gnu")))
However, other projects like QEMU have their own style preferences. So here’s what I added to use a different style for that. First, I found the qemu C style defined here. Then, to only use this on some C code, we attach a hook that only overrides the default C style if the filename contains “qemu”, an imperfect but decent-enough test.
(defconst qemu-c-style '((indent-tabs-mode . nil) (c-basic-offset . 4) (tab-width . 8) (c-comment-only-line-offset . 0) (c-hanging-braces-alist . ((substatement-open before after))) (c-offsets-alist . ((statement-block-intro . +) (substatement-open . 0) (label . 0) (statement-cont . +) (innamespace . 0) (inline-open . 0) )) (c-hanging-braces-alist . ((brace-list-open) (brace-list-intro) (brace-list-entry) (brace-list-close) (brace-entry-open) (block-close . c-snug-do-while) ;; structs have hanging braces on open (class-open . (after)) ;; ditto if statements (substatement-open . (after)) ;; and no auto newline at the end (class-close) )) ) "QEMU C Programming Style") (c-add-style "qemu" qemu-c-style) (defun maybe-qemu-style () (when (and buffer-file-name (string-match "qemu" buffer-file-name)) (c-set-style "qemu"))) (add-hook 'c-mode-hook 'maybe-qemu-style)
Since my blogging tsunami almost a month ago, I’ve been pretty quiet. The reason being that I’ve been heads down working on some new features for trinity which have turned out to be a lot more involved than I initially anticipated.
Trinity does all of its work in child processes continually forked off from a main process. For a long time I’ve had “investigate using pthreads” as a TODO item, but after various conversations at kernel summit, I decided to bump the priority of that up a little, and spend some time looking at it. I initially guessed that it would have take maybe a few weeks to have something usable, but after spending some time working on it, every time I make progress on one issue, it becomes apparent that there’s something else that is also going to need changing.
I’m taking a week off next week to clear my head and hopefully return to this work with fresh eyes, and make more progress, because so far it’s been mostly frustrating, and there may be an easier way to solve some of the problems I’ve been hitting. Sidenote: In the 15+ years I’ve been working on Linux, this is the first time I recall actually ever using pthreads in my own code. I can’t say I’ve been missing out.
Unrelated to that work, a month or so ago I came up with a band-aid fix for a problem where trinity would corrupt its own structures. That ‘fix’ turned out to break the post-mortem work I implemented a few months prior, so I’ve spent some time this week undoing that, and thinking about how I’m going to fix that properly. But before coming up with a fix, I needed to reproduce the problem reliably, and naturally now that I’ve added debug code to determine where the corruption is coming from, the bug has gone into hiding.
I need this vacation.
This is a small release; the more notable changes in man-pages-3.72 are the addition of three new pages by Peter Schiffer that document glibc commands used for memory profile and malloc tracing:
Just an FYI, I lost my GPG key a few months back during an upgrade, and have created a new one. This was signed by folk at LinuxCon/KS last month.
The new key ID / fingerprint is: D950053C / 8327 23D0 EF9D D46D 9AC9 C03C AD98 4BBF D950 053C
Please use this key and not the old one!
Have you ever played the "press any key to stop autoboot" game, followed by copying boot commands from your notes, because you wanted to keep boot loader in original (early project phases) or final (late project phases) configuration? Have you reached level 2, playing autoboot game over internet?
If so, you may want to take a look at boot shell (bs) from Not Universal Test System project. In ideal case, it knows how to turn off/on the target, break into autoboot, boot your target in development mode, and login as root when user land is ready.
Upstream Review Training Slides