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So far, we have used Harmony to implement various algorithms. But Harmony can also be used to specify what an algorithm is supposed to do. For example, Figure 8.1 specifies the intended behavior of a lock. In this case, a lock is a boolean, initially False, with two operations, acquire() and release(). The acquire() operation waits until the lock is False and then sets it to True in an atomic operation. The release() operation sets the lock back to False. The code is similar to Figure 5.3, except that waiting for the lock to become available and taking it is executed as an atomic operation.

import list
import bag

def tas(lk):
        result = !lk
        !lk = True

def cas(p, old, new):
        result = !p == old
        if result:
            !p = new

def BinSema(acquired):
    result = acquired

def Lock():
    result = BinSema(False)

def acquired(binsema):
    result = !binsema

def acquire(binsema):
    atomically when not !binsema:
        !binsema = True

def release(binsema):
    assert !binsema
    atomically !binsema = False

def held(binsema):
    result = !binsema

def Condition():
    result = bag.empty()

def wait(c, lk):
    var cnt = 0
    let (), ctx = save():
            cnt = bag.multiplicity(!c, ctx)
            !c = bag.add(!c, ctx)
            !lk = False
        atomically when (not !lk) and (bag.multiplicity(!c, ctx) <= cnt):
            !lk = True

def notify(c):
    atomically if !c != bag.empty():
        !c = bag.remove(!c, bag.bchoose(!c))

def notifyAll(c):
    atomically !c = bag.empty()

def Semaphore(cnt):
    result = cnt

def P(sema):
    atomically when !sema > 0:
        !sema -= 1

def V(sema):
    atomically !sema += 1

def Queue():
    result = []

def get(q):
    atomically when !q != []:
        result = list.head(!q)
        !q = list.tail(!q)

def put(q, item):
    atomically !q = list.append(!q, item)
Figure 8.1 (modules/synch.hny): Specification of a lock
import synch

const NTHREADS = 2

lock = synch.Lock()

def thread():
    while choose({ False, True }):
        cs: assert countLabel(cs) == 1

for i in {1..NTHREADS}:
    spawn thread()
Figure 8.2 (code/cssynch.hny): Using a lock to implement a critical section
from synch import Lock, acquire, release

sequential done

count = 0
countlock = Lock()
done = [ False, False ]

def thread(self):
    count = count + 1
    done[self] = True
    await done[1 - self]
    assert count == 2

spawn thread(0)
spawn thread(1)
Figure 8.3 (code/UpLock.hny): Figure 3.2 fixed with a lock

The code in Figure 8.1 is similar to the code in Harmony's synch module. (The module generalizes locks to binary semaphores (Chapter 16), but the lock interface is the same.) Figure 8.2 shows how locks may be used to implement a critical section. Figure 8.3 gives an example of how locks may be used to fix the program of Figure 3.2.

Note that the code of Figure 8.1 is executable in Harmony. However, the atomically keyword is not available in general programming languages and should not be used for implementation. Peterson's algorithm is an implementation of a lock, although only for two processes. In the following chapters, we will look at more general ways of implementing locks using atomic constructions that are usually available in the underlying hardware.

In Harmony, any statement can be preceded by the atomically keyword. It means that statement as a whole is to be executed atomically. The atomically keyword can be used to specify the behavior of methods such as acquire and release. But an actual executable program---such as the one in Figure 8.2---should not use the atomically keyword because---on a normal machine---it cannot be directly compiled into machine code. If we want to make the program executable on hardware, we have to show how Lock, acquire, and release are implemented, not just how they are specified. Chapter 9 presents such an implementation.

The code in Figure 8.1 also uses the Harmony when statement. A when statement waits until a time in which condition holds (not necessarily the first time) and then executes the statement block. The "await condition" statement is the same as "when condition: pass". Combined with the atomically keyword, the entire statement is executed atomically at a time that the condition holds.

It is important to appreciate the difference between an atomic section (the statements executed within an atomic block of statements) and a critical section (protected by a lock of some sort). The former ensures that while the statements are executing no other thread can execute. The latter allows multiple threads to run concurrently, just not within the critical section. The former is rarely available to a programmer (e.g., none of Python, C, or Java support it), while the latter is very common.

Atomic statements are not intended to replace locks or other synchonization primitives. When implementing synchronization solutions you should not directly use atomic statements but use the synchronization primitives that are available to you. But if you want to specify a synchronization primitive, then use atomically by all means. You can also use atomic statements in your test code. In fact, as mentioned before, assert statements are included to test if certain conditions hold in every execution and are executed atomically.