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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 a dmb memory 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() 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 <erik@rigtorp.se>.