问题
I found a fairly simple n-process mutual exclusion algorithm on page 4 (836) in the following paper:
"Mutual Exclusion Using Indivisible Reads and Writes" by Burns and Lynch
program Process_i;
type flag = (down, up);
shared var F : array [1..N] of flag;
var j : 1..N;
begin
while true do begin
1: F[i] := down;
2: remainder; (* remainder region *)
3: F[i] := down;
4: for j := 1 to i-1 do
if F[j] = up then goto 3;
5: F[i] := up;
6: for j := 1 to i-1 do
if F[j] = up then goto 3;
7: for j := i+1 to N do
if F[j] = up then goto 7;
8: critical; (* critical region *)
end
end.
I like it, because of its minimal memory use and the goto's should allow me to implement it in a method enterCriticalRegion() that returns a boolean indicating whether the process succeeded in acquiring the lock (i.e. reached line 8) or whether it hit one of the goto's and needs to try again later rather than busy-waiting. (Fairness and starvation aren't really a concern in my case)
I tried to implement this in Java and test it out by having a bunch of threads try to enter the critical region in rapid succession (looks long, but it's mostly comments):
import java.util.concurrent.atomic.AtomicInteger;
public class BurnsME {
// Variable to count processes in critical section (for verification)
private static AtomicInteger criticalCount = new AtomicInteger(0);
// shared var F : array [1..N] of flag;
private static final boolean[] F = new boolean[10000];
// Some process-local variables
private final int processID;
private boolean atLine7;
public BurnsME(int processID) {
this.processID = processID;
this.atLine7 = false;
}
/**
* Try to enter critical region.
*
* @return T - success; F - failure, need to try again later
*/
public boolean enterCriticalRegion() {
// Burns Lynch Algorithm
// Mutual Exclusion Using Indivisible Reads and Writes, p. 836
if (!atLine7) {
// 3: F[i] down
F[processID] = false;
// 4: for j:=1 to i-1 do if F[j] = up goto 3
for (int process=0; process<processID; process++)
if (F[process]) return false;
// 5: F[i] = up
F[processID] = true;
// 6: for j:=1 to i-1 do if F[j] = up goto 3
for (int process=0; process<processID; process++)
if (F[process]) return false;
atLine7 = true;
}
// 7: for j:=i+1 to N do if F[j] = up goto 7
for (int process=processID+1; process<F.length; process++)
if (F[process]) return false;
// Verify mutual exclusion
if (criticalCount.incrementAndGet()>1) {
System.err.println("TWO PROCESSES ENTERED CRITICAL SECTION!");
System.exit(1);
}
// 8: critical region
return true;
}
/**
* Leave critical region and allow next process in
*/
public void leaveCriticalRegion() {
// Reset state
atLine7 = false;
criticalCount.decrementAndGet();
// Release critical region lock
// 1: F[i] = down
F[processID] = false;
}
//===============================================================================
// Test Code
private static final int THREADS = 50;
public static void main(String[] args) {
System.out.println("Launching "+THREADS+" threads...");
for (int i=0; i<THREADS; i++) {
final int threadID = i;
new Thread() {
@Override
public void run() {
BurnsME mutex = new BurnsME(threadID);
while (true) {
if (mutex.enterCriticalRegion()) {
System.out.println(threadID+" in critical region");
mutex.leaveCriticalRegion();
}
}
}
}.start();
}
while (true);
}
}
For some reason, the mutual exclusion verification (via the AtomicInteger) keeps failing after a few seconds and the program exits with the message TWO PROCESSES ENTERED CRITICAL SECTION!.
Both the algorithm and my implementation are so simple, that I'm a little perplexed why it's not working.
Is there something wrong with the Burns/Lynch algorithm (doubt it)? Or did I make some stupid mistake somewhere that I'm just not seeing? Or is this caused by some Java instruction reordering? The latter seems somewhat unlikely to me since each assignment is followed by a potential return and should thus not be swapped with any other, no? Or is array access in Java not thread safe?
A quick aside:
Here is how I visualize the Burns and Lynch algorithm (might help think about the issue):
I'm the process and I'm standing somewhere in a row with other people (processes). When I want to enter the critical section, I do the following:
- 3/4: I look to my left and keep my hand down as long as someone there has their hand up.
- 5: If no-one to my left has their hand up, I put mine up
- 6: I check again if anyone to my left has meanwhile put their hand up. If so, I put mine back down and start over. Otherwise, I keep my hand up.
- 7: Everyone to my right goes first, so I look to my right and wait until I don't see any hands up.
- 8: Once no-one to my right has their hand up any more, I can enter the critical section.
- 1: When I'm done, I put my hand back down.
Seems solid to me... Not sure why it shouldn't work reliably...
回答1:
In the java memory model you have no guarantee that a write to F[i] will be visible to another Thread reading from it later.
The standard solution for this kind of visibility problem is to declare the shared variable as volatile, but in this case F is an array and write/reads to F[i] do not change the value of F.
It is not possible to declare an "array of volatiles ...", but one can declare F as AtomicIntegerArray and use compareAndSet to atomically change the array content without worrying about Thread-visibility.
来源：https://stackoverflow.com/questions/33270575/implementing-mutual-exclusion-algorithm-by-burns-and-lynch-in-java-could-there