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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 7.49 of the "herd7" and "klitmus7" tools must be downloaded
separately:
https://github.com/herd/herdtools7
See "herdtools7/INSTALL.md" for installation instructions.
==================
BASIC USAGE: HERD7
==================
The memory model is used, in conjunction with "herd7", to exhaustively
explore the state space of small litmus tests.
For example, to run SB+mbonceonces.litmus against the memory model:
$ herd7 -conf linux-kernel.cfg litmus-tests/SB+mbonceonces.litmus
Here is the corresponding output:
Test SB+mbonceonces 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+mbonceonces Never 0 3
Time SB+mbonceonces 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.
=====================
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+mbonceonces.litmus against hardware:
$ mkdir mymodules
$ klitmus7 -o mymodules litmus-tests/SB+mbonceonces.litmus
$ cd mymodules ; make
$ sudo sh run.sh
The corresponding output includes:
Test SB+mbonceonces 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+mbonceonces Never 0 2000000
Time SB+mbonceonces 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.
====================
DESCRIPTION OF FILES
====================
Documentation/cheatsheet.txt
Quick-reference guide to the Linux-kernel memory model.
Documentation/explanation.txt
Describes the memory model in detail.
Documentation/recipes.txt
Lists common memory-ordering patterns.
Documentation/references.txt
Provides background reading.
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.
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.
===========
LIMITATIONS
===========
The Linux-kernel memory model has the following limitations:
1. Compiler optimizations are not modeled. Of course, the use
of READ_ONCE() and WRITE_ONCE() limits the compiler's ability
to optimize, but there is Linux-kernel code that uses bare C
memory accesses. Handling this code is on the to-do list.
For more information, see Documentation/explanation.txt (in
particular, the "THE PROGRAM ORDER RELATION: po AND po-loc"
and "A WARNING" sections).
2. Multiple access sizes for a single variable are not supported,
and neither are misaligned or partially overlapping accesses.
3. Exceptions and interrupts are not modeled. In some cases,
this limitation can be overcome by modeling the interrupt or
exception with an additional process.
4. I/O such as MMIO or DMA is not supported.
5. Self-modifying code (such as that found in the kernel's
alternatives mechanism, function tracer, Berkeley Packet Filter
JIT compiler, and module loader) is not supported.
6. Complete modeling of all variants of atomic read-modify-write
operations, locking primitives, and RCU is not provided.
For example, call_rcu() and rcu_barrier() are not supported.
However, a substantial amount of support is provided for these
operations, as shown in the linux-kernel.def file.
The "herd7" tool has some additional limitations of its own, apart from
the memory model:
1. Non-trivial data structures such as arrays or structures are
not supported. However, pointers are supported, allowing trivial
linked lists to be constructed.
2. Dynamic memory allocation is not supported, although this can
be worked around in some cases by supplying multiple statically
allocated variables.
Some of these limitations may be overcome in the future, but others are
more likely to be addressed by incorporating the Linux-kernel memory model
into other tools.
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