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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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| Documentation | ||
| litmus-tests | ||
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| linux-kernel.bell | ||
| linux-kernel.cat | ||
| linux-kernel.cfg | ||
| linux-kernel.def | ||
| lock.cat | ||
| README | ||
===================================== LINUX KERNEL MEMORY CONSISTENCY MODEL ===================================== ============ INTRODUCTION ============ This directory contains the memory consistency model (memory model, for short) of the Linux kernel, written in the "cat" language and executable by the externally provided "herd7" simulator, which exhaustively explores the state space of small litmus tests. In addition, the "klitmus7" tool (also externally provided) may be used to convert a litmus test to a Linux kernel module, which in turn allows that litmus test to be exercised within the Linux kernel. ============ REQUIREMENTS ============ Version 7.52 or higher of the "herd7" and "klitmus7" tools must be downloaded separately: https://github.com/herd/herdtools7 See "herdtools7/INSTALL.md" for installation instructions. Note that although these tools usually provide backwards compatibility, this is not absolutely guaranteed. For example, a future version of herd7 might not work with the model in this release. A compatible model will likely be made available in a later release of Linux kernel. If you absolutely need to run the model in this particular release, please try using the exact version called out above. klitmus7 is independent of the model provided here. It has its own dependency on a target kernel release where converted code is built and executed. Any change in kernel APIs essential to klitmus7 will necessitate an upgrade of klitmus7. If you find any compatibility issues in klitmus7, please inform the memory model maintainers. klitmus7 Compatibility Table ---------------------------- ============ ========== target Linux herdtools7 ------------ ---------- -- 4.14 7.48 -- 4.15 -- 4.19 7.49 -- 4.20 -- 5.5 7.54 -- 5.6 -- 5.16 7.56 -- 5.17 -- 7.56.1 -- ============ ========== ================== BASIC USAGE: HERD7 ================== The memory model is used, in conjunction with "herd7", to exhaustively explore the state space of small litmus tests. Documentation describing the format, features, capabilities and limitations of these litmus tests is available in tools/memory-model/Documentation/litmus-tests.txt. Example litmus tests may be found in the Linux-kernel source tree: tools/memory-model/litmus-tests/ Documentation/litmus-tests/ Several thousand more example litmus tests are available here: https://github.com/paulmckrcu/litmus https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/herd https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/perfbook.git/tree/CodeSamples/formal/litmus Documentation describing litmus tests and now to use them may be found here: tools/memory-model/Documentation/litmus-tests.txt The remainder of this section uses the SB+fencembonceonces.litmus test located in the tools/memory-model directory. To run SB+fencembonceonces.litmus against the memory model: $ cd $LINUX_SOURCE_TREE/tools/memory-model $ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus Here is the corresponding output: Test SB+fencembonceonces Allowed States 3 0:r0=0; 1:r0=1; 0:r0=1; 1:r0=0; 0:r0=1; 1:r0=1; No Witnesses Positive: 0 Negative: 3 Condition exists (0:r0=0 /\ 1:r0=0) Observation SB+fencembonceonces Never 0 3 Time SB+fencembonceonces 0.01 Hash=d66d99523e2cac6b06e66f4c995ebb48 The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that this litmus test's "exists" clause can not be satisfied. See "herd7 -help" or "herdtools7/doc/" for more information on running the tool itself, but please be aware that this documentation is intended for people who work on the memory model itself, that is, people making changes to the tools/memory-model/linux-kernel.* files. It is not intended for people focusing on writing, understanding, and running LKMM litmus tests. ===================== BASIC USAGE: KLITMUS7 ===================== The "klitmus7" tool converts a litmus test into a Linux kernel module, which may then be loaded and run. For example, to run SB+fencembonceonces.litmus against hardware: $ mkdir mymodules $ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus $ cd mymodules ; make $ sudo sh run.sh The corresponding output includes: Test SB+fencembonceonces Allowed Histogram (3 states) 644580 :>0:r0=1; 1:r0=0; 644328 :>0:r0=0; 1:r0=1; 711092 :>0:r0=1; 1:r0=1; No Witnesses Positive: 0, Negative: 2000000 Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated Hash=d66d99523e2cac6b06e66f4c995ebb48 Observation SB+fencembonceonces Never 0 2000000 Time SB+fencembonceonces 0.16 The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate that during two million trials, the state specified in this litmus test's "exists" clause was not reached. And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/" for more information. And again, please be aware that this documentation is intended for people who work on the memory model itself, that is, people making changes to the tools/memory-model/linux-kernel.* files. It is not intended for people focusing on writing, understanding, and running LKMM litmus tests. ==================== DESCRIPTION OF FILES ==================== Documentation/README Guide to the other documents in the Documentation/ directory. linux-kernel.bell Categorizes the relevant instructions, including memory references, memory barriers, atomic read-modify-write operations, lock acquisition/release, and RCU operations. More formally, this file (1) lists the subtypes of the various event types used by the memory model and (2) performs RCU read-side critical section nesting analysis. linux-kernel.cat Specifies what reorderings are forbidden by memory references, memory barriers, atomic read-modify-write operations, and RCU. More formally, this file specifies what executions are forbidden by the memory model. Allowed executions are those which satisfy the model's "coherence", "atomic", "happens-before", "propagation", and "rcu" axioms, which are defined in the file. linux-kernel.cfg Convenience file that gathers the common-case herd7 command-line arguments. linux-kernel.def Maps from C-like syntax to herd7's internal litmus-test instruction-set architecture. litmus-tests Directory containing a few representative litmus tests, which are listed in litmus-tests/README. A great deal more litmus tests are available at https://github.com/paulmckrcu/litmus. By "representative", it means the one in the litmus-tests directory is: 1) simple, the number of threads should be relatively small and each thread function should be relatively simple. 2) orthogonal, there should be no two litmus tests describing the same aspect of the memory model. 3) textbook, developers can easily copy-paste-modify the litmus tests to use the patterns on their own code. lock.cat Provides a front-end analysis of lock acquisition and release, for example, associating a lock acquisition with the preceding and following releases and checking for self-deadlock. More formally, this file defines a performance-enhanced scheme for generation of the possible reads-from and coherence order relations on the locking primitives. README This file. scripts Various scripts, see scripts/README.