Seqlock.h
Implementation of seqlock in C++11.
A seqlock can be used as an alternative to a readers-writer lock. It will never block the writer and doesn't require any memory bus locks.
Example
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:
struct Data {
std::size_t a, b, c;
};
Seqlock<Data> sl;Store a value (can only be called from a single thread):
sl.store({1, 2, 3});Load a value (can be called from multiple threads):
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.
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. On ARM memory model you need to inserts admbmemory barrier instruction, which is not possible in C++11.
Since my target architecture is x86 I've implemented the second option:
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
- Can Seqlocks Get Along With Programming Language Memory Models? (slides)
- x86-TSO: A Rigorous and Usable Programmer’s Model for x86 Multiprocessors
- A Tutorial Introduction to the ARM and POWER Relaxed Memory Models
Testing
Testing lock-free algorithms is hard. I'm using two approaches to test the implementation:
- A test that
load()andstore()publish results in the correct order on a single thread. - A multithreaded fuzz test that
load()never see a partialstore()(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 <[email protected]>.
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