[SOLVED] CS /*

/*
* This file is derived from source code for the Pintos
* instructional operating system which is itself derived
* from the Nachos instructional operating system. The
* Nachos copyright notice is reproduced in full below.
*
* Copyright (C) 1992-1996 The Regents of the University of California.
* All rights reserved.
*
* Permission to use, copy, modify, and distribute this software
* and its documentation for any purpose, without fee, and
* without written agreement is hereby granted, provided that the
* above copyright notice and the following two paragraphs appear
* in all copies of this software.
*
* IN NO EVENT SHALL THE UNIVERSITY OF CALIFORNIA BE LIABLE TO
* ANY PARTY FOR DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR
* CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OF THIS SOFTWARE
* AND ITS DOCUMENTATION, EVEN IF THE UNIVERSITY OF CALIFORNIA
* HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* THE UNIVERSITY OF CALIFORNIA SPECIFICALLY DISCLAIMS ANY
* WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE.THE SOFTWARE PROVIDED HEREUNDER IS ON AN AS IS
* BASIS, AND THE UNIVERSITY OF CALIFORNIA HAS NO OBLIGATION TO
* PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR
* MODIFICATIONS.
*
* Modifications Copyright (C) 2017 David C. Harrison. All rights reserved.
*/

#include
#include

#include threads/condvar.h
#include threads/interrupt.h
#include threads/thread.h

/*
* Initializes condition variable COND.A condition variable
* allows one piece of code to signal a condition and cooperating
* code to receive the signal and act upon it.
*/
void
condvar_init(struct condvar *cond)
{
ASSERT(cond != NULL);
list_init(&cond->waiters);
}

/*
* Atomically releases LOCK and waits for COND to be signaled by
* some other piece of code.After COND is signaled, LOCK is
* reacquired before returning.LOCK must be held before calling
* this function.
*
* The monitor implemented by this function is Mesa style, not
* Hoare style, that is, sending and receiving a signal are not
* an atomic operation.Thus, typically the caller must recheck
* the condition after the wait completes and, if necessary, wait
* again.
*
* A given condition variable is associated with only a single
* lock, but one lock may be associated with any number of
* condition variables.That is, there is a one-to-many mapping
* from locks to condition variables.
*
* This function may sleep, so it must not be called within an
* interrupt handler.This function may be called with
* interrupts disabled, but interrupts will be turned back on if
* we need to sleep.
*/
void
condvar_wait(struct condvar *cond, struct lock *lock)
{
ASSERT(cond != NULL);
ASSERT(lock != NULL);
ASSERT(!intr_context());
ASSERT(lock_held_by_current_thread(lock));

struct semaphore waiter;
semaphore_init(&waiter, 0);
list_push_back(&cond->waiters, &waiter.elem);
lock_release(lock);
semaphore_down(&waiter);
lock_acquire(lock);
}

/*
* If any threads are waiting on COND(protected by LOCK), then
* this function signals one of them to wake up from its wait.
* LOCK must be held before calling this function.
*
* An interrupt handler cannot acquire a lock, so it does not
* make sense to try to signal a condition variable within an
* interrupt handler.
*/
void
condvar_signal(struct condvar *cond, struct lock *lock UNUSED)
{
ASSERT(cond != NULL);
ASSERT(lock != NULL);
ASSERT(!intr_context());
ASSERT(lock_held_by_current_thread(lock));

if (!list_empty(&cond->waiters)) {
semaphore_up(list_entry(list_pop_front(&cond->waiters), struct semaphore, elem));
}
}

/*
* Wakes up all threads, if any, waiting on COND(protected by
* LOCK).LOCK must be held before calling this function.
*
* An interrupt handler cannot acquire a lock, so it does not
* make sense to try to signal a condition variable within an
* interrupt handler.
*/
void
condvar_broadcast(struct condvar *cond, struct lock *lock)
{
ASSERT(cond != NULL);
ASSERT(lock != NULL);

while (!list_empty(&cond->waiters)) {
condvar_signal(cond, lock);
}
}