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sequence_lock.h (7692B)

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//
// 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_