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Stupid RCU Tricks: An Early-1970s Example

Up to now, I have been calling out Kung's and Lehman's classic 1980 “Concurrent Manipulation of Binary Search Trees” as the oldest mention of something vaguely resembling RCU.

However, while looking into the history of reference counting, I found Weizenbaum's postively antique 1963 “Symmetric List Processor (SLIP)”, which describes a list-processing library, written in FORTRAN, no less. One of its features was storage reclamation based on reference counting, in which newly reclaimed list items are added to the end of SLIP's list of available space.

And, on page 413 of his 1973 “The Art of Computer Programming: Fundamental Algorithms”, none other than Donald Knuth points out that this add-at-end implementation means that “incorrect programs run correctly for awhile”. Here, “incorrect programs” is presumably referring to readers traversing SLIP lists while failing to increment the needed SLIP reference counters.

So it is that the earliest known mention of RCU-based algorithms dismisses them all as buggy. And rightfully so, given that SLIP has no notion of anything like a grace period. Thus, Kung and Lehman remain the first known implementers of something vaguely resembling RCU—but no longer the first mention!
Suppose a Linux-kernel task registers a pair of RCU callbacks, as follows:

call_rcu(&p->rcu, myfunc);
smp_mb();
call_rcu(&q->rcu, myfunc);

Given that these two callbacks are guaranteed to be registered in order,
are they also guaranteed to be invoked in order?
Android continues to find interesting new applications and problems to solve, both within and outside the mobile arena. Mainlining continues to be an area of focus, as do a number of areas of core Android functionality, including the kernel. Other topics include efficient operation on big.LITTLE systems, support for Hikey in AOSP (and multi-device support in general), and the upcoming migration to Clang" for Android builds.

Android continues to be a very exciting and dynamic project, with the above topics merely scratching the surface. For more details, see the Android/Mobile Microconference wiki page.
After taking a break in 2015, Tracing is back at Plumbers this year! Tracing is heavily used throughout the Linux ecosystem, and provides an essential method for extracting information about the underlying code that is running on the system. Although tracing is simple in concept, effective usage and implementation can be quite involved.

Topics proposed for this year's event include new features in the BPF compiler collection, perf, and ftrace; visualization frameworks; large-scale tracing and distributed debugging; always-on analytics and monitoring; do-it-yourself tracing tools; and, last but not least, a kernel-tracing wishlist.

We hope to see you there!
This year will feature a four-fold deeper dive into checkpoint-restore technology, thanks to participation by people from a number of additional related projects! These are the OpenMPI message-passing library, Berkeley Lab Checkpoint/Restart (BLCR), and Distributed MultiThreaded CheckPointing (DMTCP) (not to be confused with TCP/IP), in addition to the Checkpoint/Restore in Userspace group that has participated in prior years.

Docker integration remains a hot topic, as is post-copy live migration, as well as testing/validation. As you might guess from the inclusion of people from BLCR and OpenMPI, checkpoint-restore for distributed workloads (rather than just single systems) is an area of interest.

Please join us for a timely and important discussion!
Testing, fuzzing, and other diagnostics have made the Linux ecosystem much more robust than in the past, but there are still embarrassing bugs. Furthermore, million-year bugs will be happening many times per day across Linux's huge installed base, so there is clearly need for even more aggressive validation.

The Testing and Fuzzing Microconference aims to significantly increase the aggression level of Linux-kernel validation, with discussions on tools and test suites including kselftest, syzkaller, trinity, mutation testing, and the 0day Test Robot. The effectiveness of these tools will be attested to by any of their victims, but we must further raise our game as the installed base of Linux continues to increase.

One additional way of raising the level of testing aggression is to document the various ABIs in machine-readable format, thus lowering the barrier to entry for new projects. Who knows? Perhaps Linux testing will be driven by artificial-intelligence techniques!

Join us for an important and spirited discussion!
Device-tree discussions are probably not quite as spirited as in the past, however, device tree is an active area. In particular, significant issues remain.

This microconference will cover the updated device-tree specification, debugging, bindings validation, and core-kernel code directions. In addition, there will be discussion of what does (and does not) go into the device tree, along with the device-creation and driver binding ordering swamp.

Join us for an important and spirited discussion!
Given that PCI was introduced more than two decades ago and that PCI Express was introduced more than ten years ago, one might think that the Linux plumbing already did everything possible to support PCI.

One would be quite wrong.

One issue with current PCI support is that resource allocation is handled on a per-architecture basis, leading to duplicate code, and, worse yet, duplicate bugs. This microconference will therefore look into possible consolidation of this code.

Another issue is integration of PCI's message-signaled interrupts (MSI) into the kernel's core interrupt code. Whilst the device tree bindings and relative core code for MSI management have just been integrated, legacy code needs more work, including architecture-specific code and PCI legacy host controller drivers. In addition, attention should be paid to the ACPI bindings, to the related ACPI kernel core code, and to the MSI passthrough usage in virtualized environments.

Advances in Virtualization technologies, System I/O devices and their memory management through IOMMU components are driving features in the PCI Express specifications (Address Translation Services (ATS) and Page Request Interface (PRI)). This microconference will also foster debate on the best way to integrate these features in the kernel in a seamless way for different architectures.

Of course, no discussion of PCI would be complete without considering firmware issues, hardware quirks, power management, and the interplay between device tree and ACPI.

Join us for an important new-to-Plumbers PCI discussion!
 

 

 

Dates:

Submissions close: September 1, 2016
Speakers notified: September 22, 2016
Slides due: November 1, 2016

We are pleased to announce the Call for Refereed-Track Proposals for the 2016 edition of the Linux Plumbers Conference, which will held be in Santa Fe, NM, USA on November 2-4 in conjunction with Linux Kernel Summit.

Refereed track presentations are 50 minutes in length and should focus on a specific aspect of the "plumbing" in the Linux system. Examples of Linux plumbing include core kernel subsystems, core libraries, windowing systems, management tools, device support, media creation/playback, and so on. The best presentations are not about finished work, but rather problems, proposals, or proof-of-concept solutions that require face-to-face discussions and debate.

Given that Plumbers is not colocated with LinuxCon this year, we are spreading the refereed-track talks over all three days. This provides a change of pace and also provides a conflict-free schedule for the refereed-track talks. (Yes, this does result in more conflicts between the referred-track talks and the Microconferences, but there never is a truly free lunch.)

Linux Plumbers Conference Program Committee members will be reviewing all submitted sessions. High-quality submisssion that cannot be accepted due to the limited number of slots will be forwarded to the Microconference leads for further consideration.

To submit a refereed track talk proposal follow the instructions at this website.

Submissions are due on or before Thursday 1 September, 2016 at 11:59PM Pacific Time. Since this is after the closure of early registration, speakers may register before this date and we'll refund the registration for any selected presentation's speaker, but for only one speaker per presentation.

Finally, we are still accepting Microconference submissions here.
Live kernel patching was accepted into the v4.0 Linux kernel in February 2015, so we can declare the 2014 LPC Live Kernel Patching Microconference to have been a roaring success! However, as was noted at the time, this is just the beginning of the real work. In short, the v4.0 work makes live kernel patching possible, but more work is required to make it more reliable and more routine.

Additional issues include stacktrace reliability, patch-safety criteria for kernel threads, thread consistency models, porting to non-x86 architectures, handling of loadable modules, compiler optimizations, userspace tooling, patching of data, automated regression testing, and patch-creation guidelines.

Join us for an important and spirited discussion!

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