77 lines
2.7 KiB
C++
77 lines
2.7 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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// http://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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// base::subtle::SpinLockDelay() and base::subtle::SpinLockWake().
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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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#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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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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for (int loop = 0; ; loop++) {
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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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r <<= 16; // 48-bit random number now in top 48-bits.
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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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// loop>>3 cannot exceed 4 because loop cannot exceed 32.
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// Select top 20..24 bits of lower 48 bits,
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// giving approximately 0ms to 16ms.
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// Mean is exponential in loop for first 32 iterations, then 8ms.
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// The futex path multiplies this by 16, since we expect explicit wakeups
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// almost always on that path.
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return r >> (44 - (loop >> 3));
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}
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} // namespace base_internal
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} // namespace absl
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