flags/internal/sequence_lock.h

//
// Copyright 2020 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_
#define ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_

#include <stddef.h>
#include <stdint.h>

#include <atomic>
#include <cassert>
#include <cstring>

#include "absl/base/optimization.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace flags_internal {

// Align 'x' up to the nearest 'align' bytes.
inline constexpr size_t AlignUp(size_t x, size_t align) {
  return align * ((x + align - 1) / align);
}

// A SequenceLock implements lock-free reads. A sequence counter is incremented
// before and after each write, and readers access the counter before and after
// accessing the protected data. If the counter is verified to not change during
// the access, and the sequence counter value was even, then the reader knows
// that the read was race-free and valid. Otherwise, the reader must fall back
// to a Mutex-based code path.
//
// This particular SequenceLock starts in an "uninitialized" state in which
// TryRead() returns false. It must be enabled by calling MarkInitialized().
// This serves as a marker that the associated flag value has not yet been
// initialized and a slow path needs to be taken.
//
// The memory reads and writes protected by this lock must use the provided
// `TryRead()` and `Write()` functions. These functions behave similarly to
// `memcpy()`, with one oddity: the protected data must be an array of
// `std::atomic<uint64>`. This is to comply with the C++ standard, which
// considers data races on non-atomic objects to be undefined behavior. See "Can
// Seqlocks Get Along With Programming Language Memory Models?"[1] by Hans J.
// Boehm for more details.
//
// [1] https://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf
class SequenceLock {
 public:
  constexpr SequenceLock() : lock_(kUninitialized) {}

  // Mark that this lock is ready for use.
  void MarkInitialized() {
    assert(lock_.load(std::memory_order_relaxed) == kUninitialized);
    lock_.store(0, std::memory_order_release);
  }

  // Copy "size" bytes of data from "src" to "dst", protected as a read-side
  // critical section of the sequence lock.
  //
  // Unlike traditional sequence lock implementations which loop until getting a
  // clean read, this implementation returns false in the case of concurrent
  // calls to `Write`. In such a case, the caller should fall back to a
  // locking-based slow path.
  //
  // Returns false if the sequence lock was not yet marked as initialized.
  //
  // NOTE: If this returns false, "dst" may be overwritten with undefined
  // (potentially uninitialized) data.
  bool TryRead(void* dst, const std::atomic<uint64_t>* src, size_t size) const {
    // Acquire barrier ensures that no loads done by f() are reordered
    // above the first load of the sequence counter.
    int64_t seq_before = lock_.load(std::memory_order_acquire);
    if (ABSL_PREDICT_FALSE(seq_before & 1) == 1) return false;
    RelaxedCopyFromAtomic(dst, src, size);
    // Another acquire fence ensures that the load of 'lock_' below is
    // strictly ordered after the RelaxedCopyToAtomic call above.
    std::atomic_thread_fence(std::memory_order_acquire);
    int64_t seq_after = lock_.load(std::memory_order_relaxed);
    return ABSL_PREDICT_TRUE(seq_before == seq_after);
  }

  // Copy "size" bytes from "src" to "dst" as a write-side critical section
  // of the sequence lock. Any concurrent readers will be forced to retry
  // until they get a read that does not conflict with this write.
  //
  // This call must be externally synchronized against other calls to Write,
  // but may proceed concurrently with reads.
  void Write(std::atomic<uint64_t>* dst, const void* src, size_t size) {
    // We can use relaxed instructions to increment the counter since we
    // are extenally synchronized. The std::atomic_thread_fence below
    // ensures that the counter updates don't get interleaved with the
    // copy to the data.
    int64_t orig_seq = lock_.load(std::memory_order_relaxed);
    assert((orig_seq & 1) == 0);  // Must be initially unlocked.
    lock_.store(orig_seq + 1, std::memory_order_relaxed);

    // We put a release fence between update to lock_ and writes to shared data.
    // Thus all stores to shared data are effectively release operations and
    // update to lock_ above cannot be re-ordered past any of them. Note that
    // this barrier is not for the fetch_add above.  A release barrier for the
    // fetch_add would be before it, not after.
    std::atomic_thread_fence(std::memory_order_release);
    RelaxedCopyToAtomic(dst, src, size);
    // "Release" semantics ensure that none of the writes done by
    // RelaxedCopyToAtomic() can be reordered after the following modification.
    lock_.store(orig_seq + 2, std::memory_order_release);
  }

  // Return the number of times that Write() has been called.
  //
  // REQUIRES: This must be externally synchronized against concurrent calls to
  // `Write()` or `IncrementModificationCount()`.
  // REQUIRES: `MarkInitialized()` must have been previously called.
  int64_t ModificationCount() const {
    int64_t val = lock_.load(std::memory_order_relaxed);
    assert(val != kUninitialized && (val & 1) == 0);
    return val / 2;
  }

  // REQUIRES: This must be externally synchronized against concurrent calls to
  // `Write()` or `ModificationCount()`.
  // REQUIRES: `MarkInitialized()` must have been previously called.
  void IncrementModificationCount() {
    int64_t val = lock_.load(std::memory_order_relaxed);
    assert(val != kUninitialized);
    lock_.store(val + 2, std::memory_order_relaxed);
  }

 private:
  // Perform the equivalent of "memcpy(dst, src, size)", but using relaxed
  // atomics.
  static void RelaxedCopyFromAtomic(void* dst, const std::atomic<uint64_t>* src,
                                    size_t size) {
    char* dst_byte = static_cast<char*>(dst);
    while (size >= sizeof(uint64_t)) {
      uint64_t word = src->load(std::memory_order_relaxed);
      std::memcpy(dst_byte, &word, sizeof(word));
      dst_byte += sizeof(word);
      src++;
      size -= sizeof(word);
    }
    if (size > 0) {
      uint64_t word = src->load(std::memory_order_relaxed);
      std::memcpy(dst_byte, &word, size);
    }
  }

  // Perform the equivalent of "memcpy(dst, src, size)", but using relaxed
  // atomics.
  static void RelaxedCopyToAtomic(std::atomic<uint64_t>* dst, const void* src,
                                  size_t size) {
    const char* src_byte = static_cast<const char*>(src);
    while (size >= sizeof(uint64_t)) {
      uint64_t word;
      std::memcpy(&word, src_byte, sizeof(word));
      dst->store(word, std::memory_order_relaxed);
      src_byte += sizeof(word);
      dst++;
      size -= sizeof(word);
    }
    if (size > 0) {
      uint64_t word = 0;
      std::memcpy(&word, src_byte, size);
      dst->store(word, std::memory_order_relaxed);
    }
  }

  static constexpr int64_t kUninitialized = -1;
  std::atomic<int64_t> lock_;
};

}  // namespace flags_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_FLAGS_INTERNAL_SEQUENCE_LOCK_H_