Repository: rigtorp/Seqlock
Branch: master
Commit: b00d8b021875
Files: 9
Total size: 16.4 KB
Directory structure:
gitextract_guyogzfb/
├── .clang-format
├── .gitignore
├── .travis.yml
├── CMakeLists.txt
├── LICENSE
├── README.md
├── include/
│ └── rigtorp/
│ └── Seqlock.h
└── src/
├── SeqlockExample.cpp
└── SeqlockTest.cpp
================================================
FILE CONTENTS
================================================
================================================
FILE: .clang-format
================================================
BasedOnStyle: LLVM
================================================
FILE: .gitignore
================================================
build
*~
.idea/
================================================
FILE: .travis.yml
================================================
language: cpp
sudo: false
matrix:
include:
# clang-4
- env: COMPILER=clang++-4.0
compiler: clang
os: linux
addons:
apt:
packages:
- clang-4.0
- libstdc++-6-dev
sources:
- ubuntu-toolchain-r-test
- llvm-toolchain-trusty-4.0
# clang-5
- env: COMPILER=clang++-5.0
compiler: clang
os: linux
addons:
apt:
packages:
- clang-5.0
- libstdc++-7-dev
sources:
- ubuntu-toolchain-r-test
- llvm-toolchain-trusty-5.0
# clang-6
- env: COMPILER=clang++-6.0
compiler: clang
os: linux
addons:
apt:
packages:
- clang-6.0
- libstdc++-7-dev
sources:
- ubuntu-toolchain-r-test
- llvm-toolchain-trusty-6.0
# gcc-5
- env: COMPILER=g++-5
compiler: gcc
os: linux
addons:
apt:
packages:
- g++-5
sources:
- ubuntu-toolchain-r-test
# gcc-6
- env: COMPILER=g++-6
compiler: gcc
os: linux
addons:
apt:
packages:
- g++-6
sources:
- ubuntu-toolchain-r-test
# gcc-7
- env: COMPILER=g++-7
compiler: gcc
os: linux
addons:
apt:
packages:
- g++-7
sources:
- ubuntu-toolchain-r-test
# gcc-8
- env: COMPILER=g++-8
compiler: gcc
os: linux
addons:
apt:
packages:
- g++-8
sources:
- ubuntu-toolchain-r-test
before_script:
- export CXX=$COMPILER
- mkdir build
- cd build
- cmake --version
- cmake ..
script: make && ctest
================================================
FILE: CMakeLists.txt
================================================
cmake_minimum_required(VERSION 3.2)
project(Seqlock CXX)
set(CMAKE_EXPORT_COMPILE_COMMANDS "true")
set(CMAKE_CXX_STANDARD 11)
add_library(Seqlock INTERFACE)
target_include_directories(Seqlock INTERFACE
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
$<INSTALL_INTERFACE:include>)
target_compile_options(Seqlock INTERFACE -Wall -Wextra -Wpedantic -Werror)
find_package(Threads REQUIRED)
add_executable(SeqlockExample src/SeqlockExample.cpp)
target_link_libraries(SeqlockExample Seqlock Threads::Threads)
add_executable(SeqlockTest src/SeqlockTest.cpp)
target_link_libraries(SeqlockTest Seqlock Threads::Threads)
enable_testing()
add_test(SeqlockTest SeqlockTest)
================================================
FILE: LICENSE
================================================
The MIT License (MIT)
Copyright (c) 2018 Erik Rigtorp <erik@rigtorp.se>
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
================================================
FILE: README.md
================================================
# Seqlock.h
[](https://travis-ci.org/rigtorp/Seqlock)
[](https://raw.githubusercontent.com/rigtorp/Seqlock/master/LICENSE)
Implementation of [seqlock](https://en.wikipedia.org/wiki/Seqlock) in
C++11.
A seqlock can be used as an alternative to
[a readers-writer lock](https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock). It
will never block the writer and doesn't require any memory bus locks.
## Example
```cpp
struct Data {
std::size_t a, b, c;
};
Seqlock<Data> sl;
sl.store({0, 0, 0});
auto t = std::thread([&] {
for (;;) {
auto d = sl.load();
if (d.a + 100 == d.b && d.c == d.a + d.b) {
return;
}
}
});
sl.store({100, 200, 300});
t.join();
```
## Usage
Create a seqlock:
```cpp
struct Data {
std::size_t a, b, c;
};
Seqlock<Data> sl;
```
Store a value (can only be called from a single thread):
```cpp
sl.store({1, 2, 3});
```
Load a value (can be called from multiple threads):
```cpp
auto v = sl.load();
```
## Implementation
Implementing the seqlock in portable C++11 is quite tricky. The basic
seqlock implementation *unconditionally loads* the sequence number,
then *unconditionally loads* the protected data and finally
*unconditionally loads* the sequence number again. Since loading the
protected data is done unconditionally on the sequence number the
compiler is free to move these loads before or after the loads from
the sequence number.
```cpp
T load() const noexcept {
T copy;
size_t seq0, seq1;
do {
seq0 = seq_.load(std::memory_order_acquire);
copy = value_;
seq1 = seq_.load(std::memory_order_acquire);
} while (seq0 != seq1 || seq0 & 1);
return copy;
}
```
Compiling this code specialized for `int` with `g++-5.2 -std=c++11 -O3` yields
the following assembly:
```
0000000000401ad0 <_ZNK7rigtorp7SeqlockIiLm64EE4loadEv>:
// copy = value_;
401ad0: 8b 47 08 mov 0x8(%rdi),%eax
401ad3: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
// do {
// seq0 = seq_.load(std::memory_order_acquire);
401ad8: 48 8b 0f mov (%rdi),%rcx
// seq1 = seq_.load(std::memory_order_acquire);
401adb: 48 8b 17 mov (%rdi),%rdx
// } while (seq0 != seq1 || seq0 & 1);
401ade: 48 39 ca cmp %rcx,%rdx
401ae1: 75 f5 jne 401ad8 <_ZNK7rigtorp7SeqlockIiLm64EE4loadEv+0x8>
401ae3: 83 e2 01 and $0x1,%edx
401ae6: 75 f0 jne 401ad8 <_ZNK7rigtorp7SeqlockIiLm64EE4loadEv+0x8>
// return copy;
401ae8: f3 c3 repz retq
401aea: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
```
We see that the compiler did indeed reorder the load of the protected
data outside the critical section and the data is no longer protected
from torn reads. Interestingly compiling using `clang++-3.7 -std=c++11
-O3` produces the correct assembly:
```
0000000000401520 <_ZNK7rigtorp7SeqlockIiLm64EE4loadEv>:
// do {
// seq0 = seq_.load(std::memory_order_acquire);
401520: 48 8b 0f mov (%rdi),%rcx
// copy = value_;
401523: 8b 47 08 mov 0x8(%rdi),%eax
// seq1 = seq_.load(std::memory_order_acquire);
401526: 48 8b 17 mov (%rdi),%rdx
// } while (seq0 != seq1 || seq0 & 1);
401529: f6 c1 01 test $0x1,%cl
40152c: 75 f2 jne 401520 <_ZNK7rigtorp7SeqlockIiLm64EE4loadEv>
40152e: 48 39 d1 cmp %rdx,%rcx
401531: 75 ed jne 401520 <_ZNK7rigtorp7SeqlockIiLm64EE4loadEv>
// return copy;
401533: c3 retq
401534: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
40153b: 00 00 00
40153e: 66 90 xchg %ax,%ax
```
There are two ways to fix this:
* Make the location of the protected data dependent on the sequence
number by storing multiple instances of the data and selecting which
one to read from based on the sequence number. This solution should
be portable to all CPU architectures, but requires extra space.
* For x86 it's enough to insert a compiler barrier using
`std::atomic_signal_fence(std::memory_order_acq_rel)`. This will
only work on the [x86 memory model][x86-mm]. On
[ARM memory model][arm-mm] you need to inserts a `dmb` memory
barrier instruction, which is not possible in C++11.
Since my target architecture is x86 I've implemented the second
option:
```cpp
T load() const noexcept {
T copy;
size_t seq0, seq1;
do {
seq0 = seq_.load(std::memory_order_acquire);
std::atomic_signal_fence(std::memory_order_acq_rel);
copy = value_;
std::atomic_signal_fence(std::memory_order_acq_rel);
seq1 = seq_.load(std::memory_order_acquire);
} while (seq0 != seq1 || seq0 & 1);
return copy;
}
```
Compiled with `g++-5.2 -std=c++11 -O3` it produces the
following correct assembly:
```
00000000004014e0 <_ZNK7rigtorp7SeqlockIiE4loadEv>:
4014e0: 48 8d 4f 08 lea 0x8(%rdi),%rcx
4014e4: 0f 1f 40 00 nopl 0x0(%rax)
// do {
// seq0 = seq_.load(std::memory_order_acquire);
// std::atomic_signal_fence(std::memory_order_acq_rel);
4014e8: 48 8b 31 mov (%rcx),%rsi
// copy = value_;
4014eb: 8b 07 mov (%rdi),%eax
// std::atomic_signal_fence(std::memory_order_acq_rel);
// seq1 = seq_.load(std::memory_order_acquire);
4014ed: 48 8b 11 mov (%rcx),%rdx
// } while (seq0 != seq1 || seq0 & 1);
4014f0: 48 39 f2 cmp %rsi,%rdx
4014f3: 75 f3 jne 4014e8 <_ZNK7rigtorp7SeqlockIiE4loadEv+0x8>
4014f5: 83 e2 01 and $0x1,%edx
4014f8: 75 ee jne 4014e8 <_ZNK7rigtorp7SeqlockIiE4loadEv+0x8>
// return copy;
4014fa: f3 c3 repz retq
4014fc: 0f 1f 40 00 nopl 0x0(%rax)
```
The store operation is implemented in a similar manner to the load
operation. Additionally the data and sequence counter is aligned and
padded to prevent false sharing with adjacent data.
References:
* [fast reader/writer lock for gettimeofday 2.5.30](http://lwn.net/Articles/7388/)
* [Can Seqlocks Get Along With Programming Language Memory Models?](http://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf)
([slides](http://safari.ece.cmu.edu/MSPC2012/slides_posters/boehm-slides.pdf))
* [x86-TSO: A Rigorous and Usable Programmer’s Model for x86 Multiprocessors][x86-mm]
* [A Tutorial Introduction to the ARM and POWER Relaxed Memory Models][arm-mm]
[x86-mm]: http://www.cl.cam.ac.uk/~pes20/weakmemory/cacm.pdf
[arm-mm]: http://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test7.pdf
## Testing
Testing lock-free algorithms is hard. I'm using two approaches to test
the implementation:
* A test that `load()` and `store()` publish results in the correct
order on a single thread.
* A multithreaded fuzz test that `load()` never see a partial
`store()` (torn read).
## Potential improvements
Allow partial updates and reads using a visitor pattern.
Support for multiple writers can be achieved by using a CAS loop to
acquire the odd sequence number in the store operation. This would
have the same effect as wrapping the seqlock in a spinlock.
Trade-off space for reduced readers-writer contention by striping
writes across multiple seqlocks.
## About
This project was created by [Erik Rigtorp](http://rigtorp.se)
<[erik@rigtorp.se](mailto:erik@rigtorp.se)>.
================================================
FILE: include/rigtorp/Seqlock.h
================================================
/*
Copyright (c) 2018 Erik Rigtorp <erik@rigtorp.se>
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/
#pragma once
#include <atomic>
#include <type_traits>
#ifndef NDEBUG
#define RIGTORP_SEQLOCK_NOINLINE __attribute__((noinline))
#else
#define RIGTORP_SEQLOCK_NOINLINE
#endif
namespace rigtorp {
template <typename T> class Seqlock {
public:
static_assert(std::is_nothrow_copy_assignable<T>::value,
"T must satisfy is_nothrow_copy_assignable");
static_assert(std::is_trivially_copy_assignable<T>::value,
"T must satisfy is_trivially_copy_assignable");
Seqlock() : seq_(0) {}
RIGTORP_SEQLOCK_NOINLINE T load() const noexcept {
T copy;
std::size_t seq0, seq1;
do {
seq0 = seq_.load(std::memory_order_acquire);
std::atomic_signal_fence(std::memory_order_acq_rel);
copy = value_;
std::atomic_signal_fence(std::memory_order_acq_rel);
seq1 = seq_.load(std::memory_order_acquire);
} while (seq0 != seq1 || seq0 & 1);
return copy;
}
RIGTORP_SEQLOCK_NOINLINE void store(const T &desired) noexcept {
std::size_t seq0 = seq_.load(std::memory_order_relaxed);
seq_.store(seq0 + 1, std::memory_order_release);
std::atomic_signal_fence(std::memory_order_acq_rel);
value_ = desired;
std::atomic_signal_fence(std::memory_order_acq_rel);
seq_.store(seq0 + 2, std::memory_order_release);
}
private:
static const std::size_t kFalseSharingRange = 128;
// Align to prevent false sharing with adjecent data
alignas(kFalseSharingRange) T value_;
std::atomic<std::size_t> seq_;
// Padding to prevent false sharing with adjecent data
char padding_[kFalseSharingRange -
((sizeof(value_) + sizeof(seq_)) % kFalseSharingRange)];
static_assert(
((sizeof(value_) + sizeof(seq_) + sizeof(padding_)) %
kFalseSharingRange) == 0,
"sizeof(Seqlock<T>) should be a multiple of kFalseSharingRange");
};
}
================================================
FILE: src/SeqlockExample.cpp
================================================
#include <rigtorp/Seqlock.h>
#include <thread>
int main(int, char *[]) {
using namespace rigtorp;
struct Data {
std::size_t a, b, c;
};
Seqlock<Data> sl;
sl.store({0, 0, 0});
auto t = std::thread([&] {
for (;;) {
auto d = sl.load();
if (d.a + 100 == d.b && d.c == d.a + d.b) {
return;
}
}
});
sl.store({100, 200, 300});
t.join();
return 0;
}
================================================
FILE: src/SeqlockTest.cpp
================================================
/*
Copyright (c) 2018 Erik Rigtorp <erik@rigtorp.se>
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/
#include <atomic>
#include <iostream>
#include <rigtorp/Seqlock.h>
#include <thread>
#include <vector>
int main(int, char *[]) {
using namespace rigtorp;
// Basic test
{
Seqlock<int> sl;
sl.store(1);
if (sl.load() != 1) {
std::terminate();
}
sl.store(2);
if (sl.load() != 2) {
std::terminate();
}
}
// Fuzz test
{
struct Data {
std::size_t a, b, c;
};
Seqlock<Data> sl;
std::atomic<std::size_t> ready(0);
std::vector<std::thread> ts;
for (int i = 0; i < 100; ++i) {
ts.push_back(std::thread([&sl, &ready]() {
while (ready == 0) {
}
for (std::size_t i = 0; i < 10000000; ++i) {
auto copy = sl.load();
if (copy.a + 100 != copy.b || copy.c != copy.a + copy.b) {
std::terminate();
}
}
ready--;
}));
}
std::size_t counter = 0;
while (true) {
Data data = {counter++, data.a + 100, data.b + data.a};
sl.store(data);
if (counter == 1) {
ready += ts.size();
}
if (ready == 0) {
break;
}
}
std::cout << counter << std::endl;
for (auto &t : ts) {
t.join();
}
}
return 0;
}
gitextract_guyogzfb/
├── .clang-format
├── .gitignore
├── .travis.yml
├── CMakeLists.txt
├── LICENSE
├── README.md
├── include/
│ └── rigtorp/
│ └── Seqlock.h
└── src/
├── SeqlockExample.cpp
└── SeqlockTest.cpp
SYMBOL INDEX (3 symbols across 3 files)
FILE: include/rigtorp/Seqlock.h
function namespace (line 34) | namespace rigtorp {
FILE: src/SeqlockExample.cpp
function main (line 4) | int main(int, char *[]) {
FILE: src/SeqlockTest.cpp
function main (line 29) | int main(int, char *[]) {
Condensed preview — 9 files, each showing path, character count, and a content snippet. Download the .json file or copy for the full structured content (18K chars).
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About this extraction
This page contains the full source code of the rigtorp/Seqlock GitHub repository, extracted and formatted as plain text for AI agents and large language models (LLMs). The extraction includes 9 files (16.4 KB), approximately 5.2k tokens, and a symbol index with 3 extracted functions, classes, methods, constants, and types. Use this with OpenClaw, Claude, ChatGPT, Cursor, Windsurf, or any other AI tool that accepts text input. You can copy the full output to your clipboard or download it as a .txt file.
Extracted by GitExtract — free GitHub repo to text converter for AI. Built by Nikandr Surkov.