82 lines
2.8 KiB
C++
82 lines
2.8 KiB
C++
// Copyright 2017 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// The OS-specific header included below must provide two calls:
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// AbslInternalSpinLockDelay() and AbslInternalSpinLockWake().
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// See spinlock_wait.h for the specs.
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#include <atomic>
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#include <cstdint>
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#include "absl/base/internal/spinlock_wait.h"
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#if defined(_WIN32)
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#include "absl/base/internal/spinlock_win32.inc"
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#elif defined(__linux__)
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#include "absl/base/internal/spinlock_linux.inc"
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#elif defined(__akaros__)
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#include "absl/base/internal/spinlock_akaros.inc"
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#else
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#include "absl/base/internal/spinlock_posix.inc"
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#endif
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace base_internal {
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// See spinlock_wait.h for spec.
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uint32_t SpinLockWait(std::atomic<uint32_t> *w, int n,
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const SpinLockWaitTransition trans[],
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base_internal::SchedulingMode scheduling_mode) {
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int loop = 0;
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for (;;) {
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uint32_t v = w->load(std::memory_order_acquire);
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int i;
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for (i = 0; i != n && v != trans[i].from; i++) {
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}
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if (i == n) {
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SpinLockDelay(w, v, ++loop, scheduling_mode); // no matching transition
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} else if (trans[i].to == v || // null transition
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w->compare_exchange_strong(v, trans[i].to,
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std::memory_order_acquire,
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std::memory_order_relaxed)) {
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if (trans[i].done) return v;
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}
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}
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}
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static std::atomic<uint64_t> delay_rand;
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// Return a suggested delay in nanoseconds for iteration number "loop"
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int SpinLockSuggestedDelayNS(int loop) {
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// Weak pseudo-random number generator to get some spread between threads
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// when many are spinning.
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uint64_t r = delay_rand.load(std::memory_order_relaxed);
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r = 0x5deece66dLL * r + 0xb; // numbers from nrand48()
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delay_rand.store(r, std::memory_order_relaxed);
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if (loop < 0 || loop > 32) { // limit loop to 0..32
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loop = 32;
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}
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const int kMinDelay = 128 << 10; // 128us
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// Double delay every 8 iterations, up to 16x (2ms).
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int delay = kMinDelay << (loop / 8);
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// Randomize in delay..2*delay range, for resulting 128us..4ms range.
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return delay | ((delay - 1) & static_cast<int>(r));
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}
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} // namespace base_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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