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authorGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
committerGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
commitc2e754829628d1e9b7a16b3389cfdace76950fdf (patch)
tree5a7f056f44e27c30e10025113b644f0b3b5801fc /absl/base/internal/spinlock_wait.cc
Initial Commit
Diffstat (limited to 'absl/base/internal/spinlock_wait.cc')
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diff --git a/absl/base/internal/spinlock_wait.cc b/absl/base/internal/spinlock_wait.cc
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+// Copyright 2017 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
+//
+// http://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.
+
+// The OS-specific header included below must provide two calls:
+// base::subtle::SpinLockDelay() and base::subtle::SpinLockWake().
+// See spinlock_wait.h for the specs.
+
+#include <atomic>
+#include <cstdint>
+
+#include "absl/base/internal/spinlock_wait.h"
+
+#if defined(_WIN32)
+#include "absl/base/internal/spinlock_win32.inc"
+#else
+#include "absl/base/internal/spinlock_posix.inc"
+#endif
+
+namespace absl {
+namespace base_internal {
+
+// See spinlock_wait.h for spec.
+uint32_t SpinLockWait(std::atomic<uint32_t> *w, int n,
+ const SpinLockWaitTransition trans[],
+ base_internal::SchedulingMode scheduling_mode) {
+ for (int loop = 0; ; loop++) {
+ uint32_t v = w->load(std::memory_order_acquire);
+ int i;
+ for (i = 0; i != n && v != trans[i].from; i++) {
+ }
+ if (i == n) {
+ SpinLockDelay(w, v, loop, scheduling_mode); // no matching transition
+ } else if (trans[i].to == v || // null transition
+ w->compare_exchange_strong(v, trans[i].to,
+ std::memory_order_acquire,
+ std::memory_order_relaxed)) {
+ if (trans[i].done) return v;
+ }
+ }
+}
+
+static std::atomic<uint64_t> delay_rand;
+
+// Return a suggested delay in nanoseconds for iteration number "loop"
+int SpinLockSuggestedDelayNS(int loop) {
+ // Weak pseudo-random number generator to get some spread between threads
+ // when many are spinning.
+ uint64_t r = delay_rand.load(std::memory_order_relaxed);
+ r = 0x5deece66dLL * r + 0xb; // numbers from nrand48()
+ delay_rand.store(r, std::memory_order_relaxed);
+
+ r <<= 16; // 48-bit random number now in top 48-bits.
+ if (loop < 0 || loop > 32) { // limit loop to 0..32
+ loop = 32;
+ }
+ // loop>>3 cannot exceed 4 because loop cannot exceed 32.
+ // Select top 20..24 bits of lower 48 bits,
+ // giving approximately 0ms to 16ms.
+ // Mean is exponential in loop for first 32 iterations, then 8ms.
+ // The futex path multiplies this by 16, since we expect explicit wakeups
+ // almost always on that path.
+ return r >> (44 - (loop >> 3));
+}
+
+} // namespace base_internal
+} // namespace absl