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103 lines
4.2 KiB
C++
103 lines
4.2 KiB
C++
// -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*-
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/* Copyright (c) 2010, Google Inc.
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions are
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* met:
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*
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* * Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* * Redistributions in binary form must reproduce the above
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* copyright notice, this list of conditions and the following disclaimer
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* in the documentation and/or other materials provided with the
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* distribution.
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* * Neither the name of Google Inc. nor the names of its
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* contributors may be used to endorse or promote products derived from
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* this software without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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// The OS-specific header included below must provide two calls:
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// base::internal::SpinLockDelay() and base::internal::SpinLockWake().
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// See spinlock_internal.h for the spec of SpinLockWake().
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// void SpinLockDelay(volatile Atomic32 *w, int32 value, int loop)
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// SpinLockDelay() generates an apprproate spin delay on iteration "loop" of a
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// spin loop on location *w, whose previously observed value was "value".
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// SpinLockDelay() may do nothing, may yield the CPU, may sleep a clock tick,
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// or may wait for a delay that can be truncated by a call to SpinlockWake(w).
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// In all cases, it must return in bounded time even if SpinlockWake() is not
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// called.
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#include "base/spinlock_internal.h"
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// forward declaration for use by spinlock_*-inl.h
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namespace base { namespace internal { static int SuggestedDelayNS(int loop); }}
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#if defined(_WIN32)
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#include "base/spinlock_win32-inl.h"
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#elif defined(__linux__)
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#include "base/spinlock_linux-inl.h"
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#else
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#include "base/spinlock_posix-inl.h"
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#endif
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namespace base {
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namespace internal {
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// Return a suggested delay in nanoseconds for iteration number "loop"
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static int SuggestedDelayNS(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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#ifdef BASE_HAS_ATOMIC64
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static base::subtle::Atomic64 rand;
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uint64 r = base::subtle::NoBarrier_Load(&rand);
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r = 0x5deece66dLL * r + 0xb; // numbers from nrand48()
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base::subtle::NoBarrier_Store(&rand, r);
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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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#else
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static Atomic32 rand;
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uint32 r = base::subtle::NoBarrier_Load(&rand);
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r = 0x343fd * r + 0x269ec3; // numbers from MSVC++
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base::subtle::NoBarrier_Store(&rand, r);
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r <<= 1; // 31-bit random number now in top 31-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 31 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 >> (12 - (loop >> 3));
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#endif
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}
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} // namespace internal
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} // namespace base
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