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A report published Sept. 24 says an investigation into a Debian package timeout uncovered a Loongson CPU erratum: the processor’s atomic add instruction could occasionally lose updates. Loongson provided test firmware that the investigators say fixed the issue, and expected a public firmware release before Oct. 1, 2026.
A Debian packaging investigation has uncovered a Loongson CPU erratum that can cause the processor’s atomic add instruction to lose updates, according to a report published Sept. 24. The investigators said Loongson supplied test firmware that resolved the failure, with a public release expected before Oct. 1, 2026.
The problem first appeared in February, when Wang Miao, a maintainer of the community Debian 13 port for LoongArch, tried to package the mathematical software normaliz on a LoongArch server. Its built-in test timed out in an infinite loop, preventing the package from completing. Other packages depend on normaliz, so the team returned to the issue rather than continuing to skip it.
The problematic code processed data points in parallel using OpenMP. It incremented a shared counter with atomic operations and stopped when the counter matched the number of points. Investigators found runs in which every point had been marked processed but the counter remained short, leaving the loop unable to exit. A second counter, also incremented atomically, did not consistently agree with the first.
The report says the compiler had generated LoongArch’s expected amadd.d instruction, and investigators did not attribute the mismatch to the program’s memory-ordering behavior. In August, they resumed the work with an AI-assisted effort to find a small reproduction. After about two days, the team had a stable reproducer and traced the lost updates to the CPU instruction. Loongson then identified a fix in roughly two weeks, according to the report.
Affects Atomic Updates on LoongArch
Atomic operations are meant to make shared updates reliable when several threads work at once. If an atomic add sometimes drops an update, software that relies on counters or similar shared values can behave incorrectly even when its source code uses the operation as intended. In this case, a counter fell behind the work completed, causing a packaging test to run indefinitely.
The report establishes the issue through a specific software failure and a reproducer; it does not establish how often the erratum occurs across workloads or which systems are affected. The reported firmware fix matters to LoongArch users because, if made generally available as described, it offers a way to address the processor behavior without requiring applications to work around the lost updates. The report says the fix has almost no performance loss, but that assessment is attributed to the investigators and is not accompanied in the supplied material by benchmark figures.
From Debian Timeout to CPU Erratum
LoongArch is the processor architecture supported by Loongson, and loong13 is a community-maintained port of Debian 13 stable for that architecture. The initial failure occurred while building normaliz, a package used by other software. The report says earlier packaging work had surfaced hidden race conditions and memory-ordering problems, which led investigators to consider software-level explanations first.
The first investigation, in February, did not produce a small test that could reliably expose the fault, so the team set the issue aside. When Wang Miao returned to it in August, the investigators changed their method: they used AI to help reduce the behavior to a minimal reproduction while a person directed the investigation. That test made it possible to identify the atomic add instruction as the source of the lost updates, according to the report.
“The CPU’s atomic add instruction occasionally fails to be atomic.”
— The report’s authors
Firmware Rollout and Hardware Scope
The report describes a fix confirmed with test firmware, but the supplied material does not confirm that a public firmware release followed. It also does not identify the affected CPU models, firmware versions, or whether every Loongson system needs an update. The frequency of lost updates in real-world software and the full range of applications that may be affected are not specified.
The investigators report almost no performance loss, but provide no measurements here. Details on the firmware’s distribution method, installation requirements, and any additional validation are also unclear from the source material.
Watch for the Firmware Release
The next reported milestone was Loongson’s planned release of updated firmware before Oct. 1, 2026. System owners and software maintainers will need the public release details to determine whether their hardware is affected and how to apply the fix. The report does not say whether a separate software workaround or further investigation is planned.
Key Questions
What did the investigation find?
The investigators traced lost counter updates in a Debian package test to a Loongson CPU’s atomic add instruction, which they said could occasionally fail to act atomically.
How was the issue discovered?
The problem first appeared as an infinite loop in normaliz while Wang Miao was packaging it for the community Debian 13 LoongArch port. In August, an AI-assisted effort helped the investigators produce a stable test that exposed the CPU erratum.
Does the firmware fix the problem?
The report’s authors say they tested Loongson’s firmware and confirmed it resolved the issue. That confirmation concerns test firmware; the supplied material does not confirm a public release.
When was the firmware expected to be released?
Loongson told the report’s authors that release was expected before Oct. 1, 2026. The report does not confirm whether that date was met.
Which Loongson CPUs are affected?
The supplied report excerpt does not identify specific processor models or firmware versions, so the affected hardware range remains unclear.
Source: hn
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