THUAI6/CAPI/cpp/grpc/include/absl/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_