The Illusive Failure-Atomic Double-Width Compare-And-Swap
Bibliographic record
Abstract
This paper explores the limitations of failure atomicity in Intel's Double-width Compare-and-Swap (DWCAS), an essential instruction for lock-free and wait-free algorithms in concurrent computing. DWCAS, known as cmpxchg16b on the Intel platform, can manipulate a pair of consecutive (and properly aligned) 8-byte words in a manner that makes updates to these memory locations visible atomically, at least in traditional systems with volatile main memories. At the same time, Intel's technical documents warn that only 8-byte stores are guaranteed to be power-fail atomic in systems equipped with non-volatile memories, meaning that such operations cannot be torn by a power failure. We review the implications of this limitation on the integrity of algorithms that assume DWCAS is both atomic in the traditional sense and also failure-atomic. Through this analysis, we aim to clarify misconceptions and advocate for robust software-based alternatives to ensure true power-fail atomicity in DWCAS operations.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.006 | 0.021 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.002 | 0.007 |
| Scholarly communication | 0.003 | 0.009 |
| Open science | 0.004 | 0.004 |
| Research integrity | 0.002 | 0.004 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".