207 lines
6.4 KiB
Plaintext
207 lines
6.4 KiB
Plaintext
=====================================
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LINUX KERNEL MEMORY CONSISTENCY MODEL
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=====================================
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============
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INTRODUCTION
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============
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This directory contains the memory consistency model (memory model, for
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short) of the Linux kernel, written in the "cat" language and executable
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by the externally provided "herd7" simulator, which exhaustively explores
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the state space of small litmus tests.
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In addition, the "klitmus7" tool (also externally provided) may be used
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to convert a litmus test to a Linux kernel module, which in turn allows
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that litmus test to be exercised within the Linux kernel.
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============
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REQUIREMENTS
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============
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Version 7.49 of the "herd7" and "klitmus7" tools must be downloaded
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separately:
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https://github.com/herd/herdtools7
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See "herdtools7/INSTALL.md" for installation instructions.
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==================
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BASIC USAGE: HERD7
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==================
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The memory model is used, in conjunction with "herd7", to exhaustively
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explore the state space of small litmus tests.
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For example, to run SB+fencembonceonces.litmus against the memory model:
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$ herd7 -conf linux-kernel.cfg litmus-tests/SB+fencembonceonces.litmus
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Here is the corresponding output:
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Test SB+fencembonceonces Allowed
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States 3
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0:r0=0; 1:r0=1;
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0:r0=1; 1:r0=0;
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0:r0=1; 1:r0=1;
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No
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Witnesses
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Positive: 0 Negative: 3
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Condition exists (0:r0=0 /\ 1:r0=0)
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Observation SB+fencembonceonces Never 0 3
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Time SB+fencembonceonces 0.01
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Hash=d66d99523e2cac6b06e66f4c995ebb48
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The "Positive: 0 Negative: 3" and the "Never 0 3" each indicate that
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this litmus test's "exists" clause can not be satisfied.
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See "herd7 -help" or "herdtools7/doc/" for more information.
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=====================
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BASIC USAGE: KLITMUS7
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=====================
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The "klitmus7" tool converts a litmus test into a Linux kernel module,
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which may then be loaded and run.
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For example, to run SB+fencembonceonces.litmus against hardware:
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$ mkdir mymodules
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$ klitmus7 -o mymodules litmus-tests/SB+fencembonceonces.litmus
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$ cd mymodules ; make
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$ sudo sh run.sh
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The corresponding output includes:
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Test SB+fencembonceonces Allowed
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Histogram (3 states)
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644580 :>0:r0=1; 1:r0=0;
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644328 :>0:r0=0; 1:r0=1;
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711092 :>0:r0=1; 1:r0=1;
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No
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Witnesses
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Positive: 0, Negative: 2000000
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Condition exists (0:r0=0 /\ 1:r0=0) is NOT validated
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Hash=d66d99523e2cac6b06e66f4c995ebb48
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Observation SB+fencembonceonces Never 0 2000000
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Time SB+fencembonceonces 0.16
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The "Positive: 0 Negative: 2000000" and the "Never 0 2000000" indicate
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that during two million trials, the state specified in this litmus
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test's "exists" clause was not reached.
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And, as with "herd7", please see "klitmus7 -help" or "herdtools7/doc/"
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for more information.
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====================
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DESCRIPTION OF FILES
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====================
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Documentation/cheatsheet.txt
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Quick-reference guide to the Linux-kernel memory model.
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Documentation/explanation.txt
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Describes the memory model in detail.
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Documentation/recipes.txt
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Lists common memory-ordering patterns.
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Documentation/references.txt
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Provides background reading.
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linux-kernel.bell
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Categorizes the relevant instructions, including memory
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references, memory barriers, atomic read-modify-write operations,
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lock acquisition/release, and RCU operations.
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More formally, this file (1) lists the subtypes of the various
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event types used by the memory model and (2) performs RCU
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read-side critical section nesting analysis.
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linux-kernel.cat
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Specifies what reorderings are forbidden by memory references,
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memory barriers, atomic read-modify-write operations, and RCU.
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More formally, this file specifies what executions are forbidden
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by the memory model. Allowed executions are those which
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satisfy the model's "coherence", "atomic", "happens-before",
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"propagation", and "rcu" axioms, which are defined in the file.
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linux-kernel.cfg
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Convenience file that gathers the common-case herd7 command-line
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arguments.
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linux-kernel.def
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Maps from C-like syntax to herd7's internal litmus-test
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instruction-set architecture.
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litmus-tests
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Directory containing a few representative litmus tests, which
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are listed in litmus-tests/README. A great deal more litmus
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tests are available at https://github.com/paulmckrcu/litmus.
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lock.cat
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Provides a front-end analysis of lock acquisition and release,
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for example, associating a lock acquisition with the preceding
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and following releases and checking for self-deadlock.
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More formally, this file defines a performance-enhanced scheme
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for generation of the possible reads-from and coherence order
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relations on the locking primitives.
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README
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This file.
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===========
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LIMITATIONS
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===========
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The Linux-kernel memory model has the following limitations:
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1. Compiler optimizations are not modeled. Of course, the use
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of READ_ONCE() and WRITE_ONCE() limits the compiler's ability
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to optimize, but there is Linux-kernel code that uses bare C
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memory accesses. Handling this code is on the to-do list.
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For more information, see Documentation/explanation.txt (in
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particular, the "THE PROGRAM ORDER RELATION: po AND po-loc"
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and "A WARNING" sections).
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2. Multiple access sizes for a single variable are not supported,
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and neither are misaligned or partially overlapping accesses.
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3. Exceptions and interrupts are not modeled. In some cases,
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this limitation can be overcome by modeling the interrupt or
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exception with an additional process.
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4. I/O such as MMIO or DMA is not supported.
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5. Self-modifying code (such as that found in the kernel's
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alternatives mechanism, function tracer, Berkeley Packet Filter
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JIT compiler, and module loader) is not supported.
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6. Complete modeling of all variants of atomic read-modify-write
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operations, locking primitives, and RCU is not provided.
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For example, call_rcu() and rcu_barrier() are not supported.
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However, a substantial amount of support is provided for these
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operations, as shown in the linux-kernel.def file.
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The "herd7" tool has some additional limitations of its own, apart from
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the memory model:
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1. Non-trivial data structures such as arrays or structures are
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not supported. However, pointers are supported, allowing trivial
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linked lists to be constructed.
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2. Dynamic memory allocation is not supported, although this can
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be worked around in some cases by supplying multiple statically
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allocated variables.
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Some of these limitations may be overcome in the future, but others are
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more likely to be addressed by incorporating the Linux-kernel memory model
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into other tools.
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