105 lines
1.8 KiB
C++
105 lines
1.8 KiB
C++
/*
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* \brief Semaphore
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* \author Norman Feske
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* \author Christian Prochaska
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* \date 2006-09-22
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*/
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/*
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* Copyright (C) 2006-2013 Genode Labs GmbH
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*
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* This file is part of the Genode OS framework, which is distributed
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* under the terms of the GNU General Public License version 2.
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*/
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#ifndef _INCLUDE__BASE__SEMAPHORE_H_
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#define _INCLUDE__BASE__SEMAPHORE_H_
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#include <base/lock.h>
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#include <util/fifo.h>
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namespace Genode { class Semaphore; }
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class Genode::Semaphore
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{
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protected:
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int _cnt;
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Lock _meta_lock;
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struct Element : Fifo<Element>::Element
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{
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Lock lock { Lock::LOCKED };
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void block() { lock.lock(); }
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void wake_up() { lock.unlock(); }
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};
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Fifo<Element> _queue;
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public:
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/**
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* Constructor
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*
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* \param n initial counter value of the semphore
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*/
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Semaphore(int n = 0) : _cnt(n) { }
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~Semaphore()
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{
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/* synchronize destruction with unfinished 'up()' */
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try { _meta_lock.lock(); } catch (...) { }
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}
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void up()
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{
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Lock::Guard lock_guard(_meta_lock);
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if (++_cnt > 0)
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return;
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/*
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* Remove element from queue and wake up the corresponding
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* blocking thread
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*/
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Element * element = _queue.dequeue();
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if (element)
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element->wake_up();
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}
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void down()
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{
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_meta_lock.lock();
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if (--_cnt < 0) {
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/*
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* Create semaphore queue element representing the thread
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* in the wait queue.
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*/
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Element queue_element;
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_queue.enqueue(&queue_element);
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_meta_lock.unlock();
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/*
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* The thread is going to block on a local lock now,
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* waiting for getting waked from another thread
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* calling 'up()'
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* */
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queue_element.block();
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} else {
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_meta_lock.unlock();
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}
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}
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/**
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* Return current semaphore counter
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*/
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int cnt() { return _cnt; }
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};
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#endif /* _INCLUDE__BASE__SEMAPHORE_H_ */
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