/*
* 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/lock.h”
#include “threads/interrupt.h”
#include “threads/thread.h”
/*
* Initializes LOCK. A lock can be held by at most a single
* thread at any given time. Our locks are not “recursive”, that
* is, it is an error for the thread currently holding a lock to
* try to acquire that lock.
*
* A lock is a specialization of a semaphore with an initial
* value of 1. The difference between a lock and such a
* semaphore is twofold. First, a semaphore can have a value
* greater than 1, but a lock can only be owned by a single
* thread at a time. Second, a semaphore does not have an owner,
* meaning that one thread can “down” the semaphore and then
* another one “up” it, but with a lock the same thread must both
* acquire and release it. When these restrictions prove
* onerous, it’s a good sign that a semaphore should be used,
* instead of a lock.
*/
void
lock_init(struct lock *lock)
{
ASSERT(lock != NULL);
lock->holder = NULL;
semaphore_init(&lock->semaphore, 1);
}
/*
* Acquires LOCK, sleeping until it becomes available if
* necessary. The lock must not already be held by the current
* thread.
*
* 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
lock_acquire(struct lock *lock)
{
ASSERT(lock != NULL);
ASSERT(!intr_context());
ASSERT(!lock_held_by_current_thread(lock));
semaphore_down(&lock->semaphore);
lock->holder = thread_current();
}
/*
* Tries to acquires LOCK and returns true if successful or false
* on failure. The lock must not already be held by the current
* thread.
*
* This function will not sleep, so it may be called within an
* interrupt handler.
*/
bool
lock_try_acquire(struct lock *lock)
{
ASSERT(lock != NULL);
ASSERT(!lock_held_by_current_thread(lock));
bool success = semaphore_try_down(&lock->semaphore);
if (success) {
lock->holder = thread_current();
}
return success;
}
/*
* Releases LOCK, which must be owned by the current thread.
*
* An interrupt handler cannot acquire a lock, so it does not
* make sense to try to release a lock within an interrupt
* handler.
*/
void
lock_release(struct lock *lock)
{
ASSERT(lock != NULL);
ASSERT(lock_held_by_current_thread(lock));
lock->holder = NULL;
semaphore_up(&lock->semaphore);
}
/*
* Returns true if the current thread holds LOCK, false otherwise.
* Note that testing whether some other thread holds a lock would be racy.
*/
bool
lock_held_by_current_thread(const struct lock *lock)
{
ASSERT(lock != NULL);
return lock->holder == thread_current();
}