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As reported by Viktor, plain accesses in LKMM are weaker than accesses to registers: the latter carry dependencies but the former do not. This is exemplified in the following snippet: int r = READ_ONCE(*x); WRITE_ONCE(*y, r); Here a data dependency links the READ_ONCE() to the WRITE_ONCE(), preserving their order, because the model treats r as a register. If r is turned into a memory location accessed by plain accesses, however, the link is broken and the order between READ_ONCE() and WRITE_ONCE() is no longer preserved. This is too conservative, since any optimizations on plain accesses that might break dependencies are also possible on registers; it also contradicts the intuitive notion of "dependency" as the data stored by the WRITE_ONCE() does depend on the data read by the READ_ONCE(), independently of whether r is a register or a memory location. This is resolved by redefining all dependencies to include dependencies carried by memory accesses; a dependency is said to be carried by memory accesses (in the model: carry-dep) from one load to another load if the initial load is followed by an arbitrarily long sequence alternating between stores and loads of the same thread, where the data of each store depends on the previous load, and is read by the next load. Any dependency linking the final load in the sequence to another access also links the initial load in the sequence to that access. More deep details can be found in this LKML discussion: https://lore.kernel.org/lkml/d86295788ad14a02874ab030ddb8a6f8@huawei.com/ Reported-by: Viktor Vafeiadis <viktor@mpi-sws.org> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huawei.com> Reviewed-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Paul E. McKenney <paulmck@kernel.org> |
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| .. | ||
| access-marking.txt | ||
| cheatsheet.txt | ||
| control-dependencies.txt | ||
| explanation.txt | ||
| glossary.txt | ||
| litmus-tests.txt | ||
| ordering.txt | ||
| README | ||
| recipes.txt | ||
| references.txt | ||
| simple.txt | ||
It has been said that successful communication requires first identifying what your audience knows and then building a bridge from their current knowledge to what they need to know. Unfortunately, the expected Linux-kernel memory model (LKMM) audience might be anywhere from novice to expert both in kernel hacking and in understanding LKMM. This document therefore points out a number of places to start reading, depending on what you know and what you would like to learn. Please note that the documents later in this list assume that the reader understands the material provided by documents earlier in this list. o You are new to Linux-kernel concurrency: simple.txt o You have some background in Linux-kernel concurrency, and would like an overview of the types of low-level concurrency primitives that the Linux kernel provides: ordering.txt Here, "low level" means atomic operations to single variables. o You are familiar with the Linux-kernel concurrency primitives that you need, and just want to get started with LKMM litmus tests: litmus-tests.txt o You are familiar with Linux-kernel concurrency, and would like a detailed intuitive understanding of LKMM, including situations involving more than two threads: recipes.txt o You would like a detailed understanding of what your compiler can and cannot do to control dependencies: control-dependencies.txt o You are familiar with Linux-kernel concurrency and the use of LKMM, and would like a quick reference: cheatsheet.txt o You are familiar with Linux-kernel concurrency and the use of LKMM, and would like to learn about LKMM's requirements, rationale, and implementation: explanation.txt o You are interested in the publications related to LKMM, including hardware manuals, academic literature, standards-committee working papers, and LWN articles: references.txt ==================== DESCRIPTION OF FILES ==================== README This file. cheatsheet.txt Quick-reference guide to the Linux-kernel memory model. control-dependencies.txt Guide to preventing compiler optimizations from destroying your control dependencies. explanation.txt Detailed description of the memory model. litmus-tests.txt The format, features, capabilities, and limitations of the litmus tests that LKMM can evaluate. ordering.txt Overview of the Linux kernel's low-level memory-ordering primitives by category. recipes.txt Common memory-ordering patterns. references.txt Background information. simple.txt Starting point for someone new to Linux-kernel concurrency. And also a reminder of the simpler approaches to concurrency!