Source/bmalloc/bmalloc/StaticMutex.cpp

/*
 * Copyright (C) 2014 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
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 */

#include "ScopeExit.h"
#include "StaticMutex.h"
#include <thread>

namespace bmalloc {

void StaticMutex::lockSlowCase()
{
    // The longest critical section in bmalloc is much shorter than the
    // time it takes to make a system call to yield to the OS scheduler.
    // So, we try again a lot before we yield.
    static const size_t aLot = 256;
    
    if (!m_isSpinning.test_and_set()) {
        auto clear = makeScopeExit([&] { m_isSpinning.clear(); });

        for (size_t i = 0; i < aLot; ++i) {
            if (try_lock())
                return;
        }
    }

    // Avoid spinning pathologically.
    while (!try_lock())
        std::this_thread::yield();
}

} // namespace bmalloc