| ===================================== |
| 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. |
