When a kernel subsystem maintainer has a set of commits to send up the chain toward the mainline, the git request-pull command is usually the right tool for the job. But various maintainers have noticed over the years that this command can sometimes generate confusing results when confronted with anything but the simplest of histories. A brief conversation on the linux-kernel mailing list delved into why this situation comes about and what maintainers can do in response.
The 5.4-rc4 kernel prepatch is out for testing. “This release cycle remains pretty normal. In fact, the rc’s have been a bit on the smaller side of the average of the last few releases, and rc4 continues this, if only barely.”
kmalloc() is a frequently used primitive for the allocation of small objects in the kernel. During the 2019 Linux Storage, Filesystem, and Memory Management Summit, Vlastimil Babka led a session about the unexpected alignment problems developers face when using this function. After a few months he has come back with the second version of a patch set implementing a natural alignment guarantee for kmalloc(). From the strong opposition it faced initially, it seemed that the change would not get accepted. However, it ended up in Linus Torvalds’s tree. Let’s explore what happened.
The final days of the 5.3 kernel development cycle included an extensive discussion of the getrandom() API and the reversion of an ext4 improvement that was indirectly causing boot hangs due to a lack of entropy. Blocking filesystem improvements because they are too effective is clearly not a good long-term development strategy for the kernel, so there was a consensus that some sort of better solution had to be found. What was lacking was an idea of what that solution should be. It is thus surprising that the problem appears to have been dealt with in 5.4 with little in the way of dissent or disagreement.
Google has announced version 1.0 of its Bazel build system. “A growing list of Bazel users attests to the widespread demand for scalable, reproducible, and multi-lingual builds. Bazel helps Google be more open too: several large Google open source projects, such as Angular and TensorFlow, use Bazel. Users have reported 3x test time reductions and 10x faster build speeds after switching to Bazel.”
The KDE project has announced the release of version 5.17 of the Plasma desktop environment. “Night Color, the color-grading system that relaxes your eyes when the sun sets, has landed for X11. Your Plasma desktop also recognizes when you are giving a presentation, and stops messages popping up in the middle of your slideshow. If you are using Wayland, Plasma now comes with fractional scaling, which means that you can adjust the size of all your desktop elements, windows, fonts and panels perfectly to your HiDPI monitor.”
Version 7.2 of PyPy, an implementation of the Python language, is out. With this release, Python 3.6 release is deemed ready: “This release removes the ‘beta’ tag from PyPy3.6. While there may still be some small corner-case incompatibilities (around the exact error messages in exceptions and the handling of faulty codec errorhandlers) we are happy with the quality of the 3.6 series and are looking forward to working on a Python 3.7 interpreter.”
Race conditions can be some of the trickiest bugs to find. The resulting problems can be subtle, and reproducing the problem in order to track it down can be difficult or impossible; often code inserted to narrow down a race condition will cause it to stop manifesting entirely. A tool that can find race conditions automatically would thus be a valuable thing for the kernel community to have. In late September, Marco Elver announced a tool called KCSAN (the Kernel Concurrency Sanitizer) that does exactly that — and which has already found a number of real problems.
The 5.4-rc3 kernel prepatch is out for testing. “Things continue to look fairly normal, with rc3 being larger than rc2, as people are starting to find more regressions, but 5.4 so far remains on the smaller side of recent releases.”
As noted earlier, when compiling Linux-kernel code that does a plain C-language load or store, as in “a=b“, the C standard grants the compiler the right to assume that the affected variables are neither accessed nor modified by any other thread at the time of that load or store. The compiler is therefore permitted to carry out a surprisingly large number of optimizations, any number of which might ruin your concurrent code’s day. Given that current compilers usually do not emit diagnostics warning of potential ruined days, it would be good to have other tools take on this task.
It is no secret that much of the work on the in-kernel BPF virtual machine and associated user-space support code is being done at Facebook. But less is known about how Facebook is actually using BPF. At Kernel Recipes 2019, BPF developer Alexei Starovoitov described a bit of that work, though even he admitted that he didn’t know what most of the BPF programs running there were doing. He also summarized recent developments with BPF and some near-future work.
OpenSSH 8.1 is out. It includes some security fixes, including the encryption of keys at rest to defend them against speculative-execution attacks. There is also an experimental new signature and verification mechanism for public keys.
One of the many changes in the 5.4 kernel is the completion (insofar as anything in the kernel is truly complete) of the pidfd API. Getting that work done has been “a wild ride so far”, according to its author Christian Brauner during a session at the 2019 Kernel Recipes conference. He went on to describe the history of this work and some lessons for others interested in adding major new APIs to the Linux kernel.
While Richard Stallman has resigned from the Free Software Foundation and MIT, he continues to hold onto his position as the head of the GNU project. Now, the FSF has announced that it is “working with GNU leadership on a shared understanding of the relationship for the future” and is seeking comments from the community on what that should be.
Meanwhile, a group of maintainers for specific GNU project has posted a joint statement calling for new leadership at GNU. “We believe that Richard Stallman cannot represent all of GNU. We think it is now time for GNU maintainers to collectively decide about the organization of the project. The GNU Project we want to build is one that everyone can trust to defend their freedom.”
The second 5.4 kernel prepatch is out for testing. “So nothing looks particularly worrisome, but usually rc2 is fairly calm and it takes a while for any regressions to be noticed.” This release also changes the code name to “Nesting Opossum”.
The 5.3.4, 5.2.19, 4.19.77, 4.14.147, 4.9.195, and 4.4.195 stable kernel updates have all been released; each contains a relatively large set of important fixes and updates.
The collective thoughts of the interwebz
By continuing to use the site, you agree to the use of cookies. more information
The cookie settings on this website are set to "allow cookies" to give you the best browsing experience possible. If you continue to use this website without changing your cookie settings or you click "Accept" below then you are consenting to this.