My app consist of the main-process and two threads, all running concurrently and making use of three fifo-queues:
The fifo-q's are Qmain, Q1 and Q2. Internally the queues each use a counter that is incremented when an item is put into the queue, and decremented when an item is 'get'ed from the queue.
The processing involve two threads,
QMaster, which get from Q1 and Q2, and put into Qmain,
Monitor, which put into Q2,
and the main process, which get from Qmain and put into Q1.
The QMaster-thread loop consecutively checks the counts of Q1 and Q2 and if any items are in the q's, it get's them and puts them into Qmain.
The Monitor-thread loop obtains data from external sources, package it and put it into Q2.
The main-process of the app also runs a loop checking the count of Qmain, and if any items, get's an item
from Qmain at each iteration of the loop and process it further. During this processing it occasionally
puts an item into Q1 to be processed later (when it is get'ed from Qmain in turn).
The problem:
I've implemented all as described above, and it works for a randomly (short) time and then hangs.
I've managed to identify the source of the crashing to happen in the increment/decrement of the
count of a fifo-q (it may happen in any of them).
What I've tried:
Using three mutex's: QMAIN_LOCK, Q1_LOCK and Q2_LOCK, which I lock whenever any get/put operation
is done on a relevant fifo-q. Result: the app doesn't get going, just hangs.
The main-process must continue running all the time, must not be blocked on a 'read' (named-pipes fail, socketpair fail).
Any advice?
I think I'm not implementing the mutex's properly, how should it be done?
(Any comments on improving the above design also welcome)
[edit] below are the processes and the fifo-q-template:
Where & how in this should I place the mutex's to avoid the problems described above?
main-process:
...
start thread QMaster
start thread Monitor
...
while (!quit)
{
...
if (Qmain.count() > 0)
{
X = Qmain.get();
process(X)
delete X;
}
...
//at some random time:
Q2.put(Y);
...
}
Monitor:
{
while (1)
{
//obtain & package data
Q2.put(data)
}
}
QMaster:
{
while(1)
{
if (Q1.count() > 0)
Qmain.put(Q1.get());
if (Q2.count() > 0)
Qmain.put(Q2.get());
}
}
fifo_q:
template < class X* > class fifo_q
{
struct item
{
X* data;
item *next;
item() { data=NULL; next=NULL; }
}
item *head, *tail;
int count;
public:
fifo_q() { head=tail=NULL; count=0; }
~fifo_q() { clear(); /*deletes all items*/ }
void put(X x) { item i=new item(); (... adds to tail...); count++; }
X* get() { X *d = h.data; (...deletes head ...); count--; return d; }
clear() {...}
};
An example of how I would adapt the design and lock the queue access the posix way.
Remark that I would wrap the mutex to use RAII or use boost-threading and that I would use stl::deque or stl::queue as queue, but staying as close as possible to your code:
main-process:
...
start thread Monitor
...
while (!quit)
{
...
if (Qmain.count() > 0)
{
X = Qmain.get();
process(X)
delete X;
}
...
//at some random time:
QMain.put(Y);
...
}
Monitor:
{
while (1)
{
//obtain & package data
QMain.put(data)
}
}
fifo_q:
template < class X* > class fifo_q
{
struct item
{
X* data;
item *next;
item() { data=NULL; next=NULL; }
}
item *head, *tail;
int count;
pthread_mutex_t m;
public:
fifo_q() { head=tail=NULL; count=0; }
~fifo_q() { clear(); /*deletes all items*/ }
void put(X x)
{
pthread_mutex_lock(&m);
item i=new item();
(... adds to tail...);
count++;
pthread_mutex_unlock(&m);
}
X* get()
{
pthread_mutex_lock(&m);
X *d = h.data;
(...deletes head ...);
count--;
pthread_mutex_unlock(&m);
return d;
}
clear() {...}
};
Remark too that the mutex still needs to be initialized as in the example here and that count() should also use the mutex
Use the debugger. When your solution with mutexes hangs look at what the threads are doing and you will get a good idea about the cause of the problem.
What is your platform? In Unix/Linux you can use POSIX message queues (you can also use System V message queues, sockets, FIFOs, ...) so you don't need mutexes.
Learn about condition variables. By your description it looks like your Qmaster-thread is busy looping, burning your CPU.
One of your responses suggest you are doing something like:
Q2_mutex.lock()
Qmain_mutex.lock()
Qmain.put(Q2.get())
Qmain_mutex.unlock()
Q2_mutex.unlock()
but you probably want to do it like:
Q2_mutex.lock()
X = Q2.get()
Q2_mutex.unlock()
Qmain_mutex.lock()
Qmain.put(X)
Qmain_mutex.unlock()
and as Gregory suggested above, encapsulate the logic into the get/put.
EDIT: Now that you posted your code I wonder, is this a learning exercise?
Because I see that you are coding your own FIFO queue class instead of using the C++ standard std::queue. I suppose you have tested your class really well and the problem is not there.
Also, I don't understand why you need three different queues. It seems that the Qmain queue would be enough, and then you will not need the Qmaster thread that is indeed busy waiting.
About the encapsulation, you can create a synch_fifo_q class that encapsulates the fifo_q class. Add a private mutex variable and then the public methods (put, get, clear, count,...) should be like put(X) { lock m_mutex; m_fifo_q.put(X); unlock m_mutex; }
question: what would happen if you have more than one reader from the queue? Is it guaranteed that after a "count() > 0" you can do a "get()" and get an element?
I wrote a simple application below:
#include <queue>
#include <windows.h>
#include <process.h>
using namespace std;
queue<int> QMain, Q1, Q2;
CRITICAL_SECTION csMain, cs1, cs2;
unsigned __stdcall TMaster(void*)
{
while(1)
{
if( Q1.size() > 0)
{
::EnterCriticalSection(&cs1);
::EnterCriticalSection(&csMain);
int i1 = Q1.front();
Q1.pop();
//use i1;
i1 = 2 * i1;
//end use;
QMain.push(i1);
::LeaveCriticalSection(&csMain);
::LeaveCriticalSection(&cs1);
}
if( Q2.size() > 0)
{
::EnterCriticalSection(&cs2);
::EnterCriticalSection(&csMain);
int i1 = Q2.front();
Q2.pop();
//use i1;
i1 = 3 * i1;
//end use;
QMain.push(i1);
::LeaveCriticalSection(&csMain);
::LeaveCriticalSection(&cs2);
}
}
return 0;
}
unsigned __stdcall TMoniter(void*)
{
while(1)
{
int irand = ::rand();
if ( irand % 6 >= 3)
{
::EnterCriticalSection(&cs2);
Q2.push(irand % 6);
::LeaveCriticalSection(&cs2);
}
}
return 0;
}
unsigned __stdcall TMain(void)
{
while(1)
{
if (QMain.size() > 0)
{
::EnterCriticalSection(&cs1);
::EnterCriticalSection(&csMain);
int i = QMain.front();
QMain.pop();
i = 4 * i;
Q1.push(i);
::LeaveCriticalSection(&csMain);
::LeaveCriticalSection(&cs1);
}
}
return 0;
}
int _tmain(int argc, _TCHAR* argv[])
{
::InitializeCriticalSection(&cs1);
::InitializeCriticalSection(&cs2);
::InitializeCriticalSection(&csMain);
unsigned threadID;
::_beginthreadex(NULL, 0, &TMaster, NULL, 0, &threadID);
::_beginthreadex(NULL, 0, &TMoniter, NULL, 0, &threadID);
TMain();
return 0;
}
You should not lock second mutex when you already locked one.
Since the question is tagged with C++, I suggest to implement locking inside get/add logic of the queue class (e.g. using boost locks) or write a wrapper if your queue is not a class.
This allows you to simplify the locking logic.
Regarding the sources you have added: queue size check and following put/get should be done in one transaction otherwise another thread can edit the queue in between
Are you acquiring multiple locks simultaneously? This is generally something you want to avoid. If you must, ensure you are always acquiring the locks in the same order in each thread (this is more restrictive to your concurrency and why you generally want to avoid it).
Other concurrency advice: Are you acquiring the lock prior to reading the queue sizes? If you're using a mutex to protect the queues, then your queue implementation isn't concurrent and you probably need to acquire the lock before reading the queue size.
1 problem may occur due to this rule "The main-process must continue running all the time, must not be blocked on a 'read'". How did you implement it? what is the difference between 'get' and 'read'?
Problem seems to be in your implementation, not in the logic. And as you stated, you should not be in any dead lock because you are not acquiring another lock whether in a lock.
I understand what livelock is, but I was wondering if anyone had a good code-based example of it? And by code-based, I do not mean "two people trying to get past each other in a corridor". If I read that again, I'll lose my lunch.
Here's a very simple Java example of livelock where a husband and wife are trying to eat soup, but only have one spoon between them. Each spouse is too polite, and will pass the spoon if the other has not yet eaten.
public class Livelock {
static class Spoon {
private Diner owner;
public Spoon(Diner d) { owner = d; }
public Diner getOwner() { return owner; }
public synchronized void setOwner(Diner d) { owner = d; }
public synchronized void use() {
System.out.printf("%s has eaten!", owner.name);
}
}
static class Diner {
private String name;
private boolean isHungry;
public Diner(String n) { name = n; isHungry = true; }
public String getName() { return name; }
public boolean isHungry() { return isHungry; }
public void eatWith(Spoon spoon, Diner spouse) {
while (isHungry) {
// Don't have the spoon, so wait patiently for spouse.
if (spoon.owner != this) {
try { Thread.sleep(1); }
catch(InterruptedException e) { continue; }
continue;
}
// If spouse is hungry, insist upon passing the spoon.
if (spouse.isHungry()) {
System.out.printf(
"%s: You eat first my darling %s!%n",
name, spouse.getName());
spoon.setOwner(spouse);
continue;
}
// Spouse wasn't hungry, so finally eat
spoon.use();
isHungry = false;
System.out.printf(
"%s: I am stuffed, my darling %s!%n",
name, spouse.getName());
spoon.setOwner(spouse);
}
}
}
public static void main(String[] args) {
final Diner husband = new Diner("Bob");
final Diner wife = new Diner("Alice");
final Spoon s = new Spoon(husband);
new Thread(new Runnable() {
public void run() { husband.eatWith(s, wife); }
}).start();
new Thread(new Runnable() {
public void run() { wife.eatWith(s, husband); }
}).start();
}
}
Run the program and you'll get:
Bob: You eat first my darling Alice!
Alice: You eat first my darling Bob!
Bob: You eat first my darling Alice!
Alice: You eat first my darling Bob!
Bob: You eat first my darling Alice!
Alice: You eat first my darling Bob!
...
This will go on forever if uninterrupted. This is a livelock because both Alice and Bob are repeatedly asking each other to go first in an infinite loop (hence live). In a deadlock situation, both Alice and Bob would simply be frozen waiting on each other to go first — they won't be doing anything except wait (hence dead).
Flippant comments aside, one example which is known to come up is in code which tries to detect and handle deadlock situations. If two threads detect a deadlock, and try to "step aside" for each other, without care they will end up being stuck in a loop always "stepping aside" and never managing to move forwards.
By "step aside" I mean that they would release the lock and attempt to let the other one acquire it. We might imagine the situation with two threads doing this (pseudocode):
// thread 1
getLocks12(lock1, lock2)
{
lock1.lock();
while (lock2.locked())
{
// attempt to step aside for the other thread
lock1.unlock();
wait();
lock1.lock();
}
lock2.lock();
}
// thread 2
getLocks21(lock1, lock2)
{
lock2.lock();
while (lock1.locked())
{
// attempt to step aside for the other thread
lock2.unlock();
wait();
lock2.lock();
}
lock1.lock();
}
Race conditions aside, what we have here is a situation where both threads, if they enter at the same time will end up running in the inner loop without proceeding. Obviously this is a simplified example. A naiive fix would be to put some kind of randomness in the amount of time the threads would wait.
The proper fix is to always respect the lock heirarchy. Pick an order in which you acquire the locks and stick to that. For example if both threads always acquire lock1 before lock2, then there is no possibility of deadlock.
As there is no answer marked as accepted answer, I have attempted to create live lock example;
Original program was written by me in Apr 2012 to learn various concept of multithreading. This time I have modified it to create deadlock, race condition, livelock etc.
So let's understand the problem statement first;
Cookie Maker Problem
There are some ingredient containers: ChocoPowederContainer, WheatPowderContainer. CookieMaker takes some amount of powder from ingredient containers to bake a Cookie. If a cookie maker finds a container empty it checks for another container to save time. And waits until Filler fills the required container. There is a Filler who checks container on regular interval and fills some quantity if a container needs it.
Please check the complete code on github;
Let me explain you implementation in brief.
I start Filler as daemon thread. So it'll keep filling containers on regular interval. To fill a container first it locks the container -> check if it needs some powder -> fills it -> signal all makers who are waiting for it -> unlock container.
I create CookieMaker and set that it can bake up to 8 cookies in parallel. And I start 8 threads to bake cookies.
Each maker thread creates 2 callable sub-thread to take powder from containers.
sub-thread takes a lock on a container and check if it has enough powder. If not, wait for some time. Once Filler fills the container, it takes the powder, and unlock the container.
Now it completes other activities like: making mixture and baking etc.
Let's have a look in the code:
CookieMaker.java
private Integer getMaterial(final Ingredient ingredient) throws Exception{
:
container.lock();
while (!container.getIngredient(quantity)) {
container.empty.await(1000, TimeUnit.MILLISECONDS);
//Thread.sleep(500); //For deadlock
}
container.unlock();
:
}
IngredientContainer.java
public boolean getIngredient(int n) throws Exception {
:
lock();
if (quantityHeld >= n) {
TimeUnit.SECONDS.sleep(2);
quantityHeld -= n;
unlock();
return true;
}
unlock();
return false;
}
Everything runs fine until Filler is filling the containers. But if I forget to start the filler, or filler goes on unexpected leave, sub-threads keep changing their states to allow other maker to go and check the container.
I have also create a daemon ThreadTracer which keeps watch on thread states and deadlocks. This the output from console;
2016-09-12 21:31:45.065 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:RUNNABLE, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:TIMED_WAITING]
2016-09-12 21:31:45.065 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:TIMED_WAITING, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:TIMED_WAITING]
WheatPowder Container has 0 only.
2016-09-12 21:31:45.082 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:TIMED_WAITING, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:RUNNABLE]
2016-09-12 21:31:45.082 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:TIMED_WAITING, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:TIMED_WAITING]
You'll notice that sub-threads and changing their states and waiting.
A real (albeit without exact code) example is two competing processes live locking in an attempt to correct for a SQL server deadlock, with each process using the same wait-retry algorithm for retrying. While it's the luck of timing, I have seen this happen on separate machines with similar performance characteristics in response to a message added to an EMS topic (e.g. saving an update of a single object graph more than once), and not being able to control the lock order.
A good solution in this case would be to have competing consumers (prevent duplicate processing as high up in the chain as possible by partitioning the work on unrelated objects).
A less desirable (ok, dirty-hack) solution is to break the timing bad luck (kind of force differences in processing) in advance or break it after deadlock by using different algorithms or some element of randomness. This could still have issues because its possible the lock taking order is "sticky" for each process, and this takes a certain minimum of time not accounted for in the wait-retry.
Yet another solution (at least for SQL Server) is to try a different isolation level (e.g. snapshot).
I coded up the example of 2 persons passing in a corridor. The two threads will avoid each other as soon as they realise their directions are the same.
public class LiveLock {
public static void main(String[] args) throws InterruptedException {
Object left = new Object();
Object right = new Object();
Pedestrian one = new Pedestrian(left, right, 0); //one's left is one's left
Pedestrian two = new Pedestrian(right, left, 1); //one's left is two's right, so have to swap order
one.setOther(two);
two.setOther(one);
one.start();
two.start();
}
}
class Pedestrian extends Thread {
private Object l;
private Object r;
private Pedestrian other;
private Object current;
Pedestrian (Object left, Object right, int firstDirection) {
l = left;
r = right;
if (firstDirection==0) {
current = l;
}
else {
current = r;
}
}
void setOther(Pedestrian otherP) {
other = otherP;
}
Object getDirection() {
return current;
}
Object getOppositeDirection() {
if (current.equals(l)) {
return r;
}
else {
return l;
}
}
void switchDirection() throws InterruptedException {
Thread.sleep(100);
current = getOppositeDirection();
System.out.println(Thread.currentThread().getName() + " is stepping aside.");
}
public void run() {
while (getDirection().equals(other.getDirection())) {
try {
switchDirection();
Thread.sleep(100);
} catch (InterruptedException e) {}
}
}
}
C# version of jelbourn's code:
using System;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
namespace LiveLockExample
{
static class Program
{
public static void Main(string[] args)
{
var husband = new Diner("Bob");
var wife = new Diner("Alice");
var s = new Spoon(husband);
Task.WaitAll(
Task.Run(() => husband.EatWith(s, wife)),
Task.Run(() => wife.EatWith(s, husband))
);
}
public class Spoon
{
public Spoon(Diner diner)
{
Owner = diner;
}
public Diner Owner { get; private set; }
[MethodImpl(MethodImplOptions.Synchronized)]
public void SetOwner(Diner d) { Owner = d; }
[MethodImpl(MethodImplOptions.Synchronized)]
public void Use()
{
Console.WriteLine("{0} has eaten!", Owner.Name);
}
}
public class Diner
{
public Diner(string n)
{
Name = n;
IsHungry = true;
}
public string Name { get; private set; }
private bool IsHungry { get; set; }
public void EatWith(Spoon spoon, Diner spouse)
{
while (IsHungry)
{
// Don't have the spoon, so wait patiently for spouse.
if (spoon.Owner != this)
{
try
{
Thread.Sleep(1);
}
catch (ThreadInterruptedException e)
{
}
continue;
}
// If spouse is hungry, insist upon passing the spoon.
if (spouse.IsHungry)
{
Console.WriteLine("{0}: You eat first my darling {1}!", Name, spouse.Name);
spoon.SetOwner(spouse);
continue;
}
// Spouse wasn't hungry, so finally eat
spoon.Use();
IsHungry = false;
Console.WriteLine("{0}: I am stuffed, my darling {1}!", Name, spouse.Name);
spoon.SetOwner(spouse);
}
}
}
}
}
Consider a UNIX system having 50 process slots.
Ten programs are running, each of which having to create 6 (sub)processes.
After each process has created 4 processes, the 10 original processes and the 40 new processes have exhausted the table. Each of the 10 original processes now sits in an endless loop forking and failing – which is aptly the situation of a livelock. The probability of this happening is very little but it could happen.
One example here might be using a timed tryLock to obtain more than one lock and if you can't obtain them all, back off and try again.
boolean tryLockAll(Collection<Lock> locks) {
boolean grabbedAllLocks = false;
for(int i=0; i<locks.size(); i++) {
Lock lock = locks.get(i);
if(!lock.tryLock(5, TimeUnit.SECONDS)) {
grabbedAllLocks = false;
// undo the locks I already took in reverse order
for(int j=i-1; j >= 0; j--) {
lock.unlock();
}
}
}
}
I could imagine such code would be problematic as you have lots of threads colliding and waiting to obtain a set of locks. But I'm not sure this is very compelling to me as a simple example.
Python version of jelbourn's code:
import threading
import time
lock = threading.Lock()
class Spoon:
def __init__(self, diner):
self.owner = diner
def setOwner(self, diner):
with lock:
self.owner = diner
def use(self):
with lock:
"{0} has eaten".format(self.owner)
class Diner:
def __init__(self, name):
self.name = name
self.hungry = True
def eatsWith(self, spoon, spouse):
while(self.hungry):
if self != spoon.owner:
time.sleep(1) # blocks thread, not process
continue
if spouse.hungry:
print "{0}: you eat first, {1}".format(self.name, spouse.name)
spoon.setOwner(spouse)
continue
# Spouse was not hungry, eat
spoon.use()
print "{0}: I'm stuffed, {1}".format(self.name, spouse.name)
spoon.setOwner(spouse)
def main():
husband = Diner("Bob")
wife = Diner("Alice")
spoon = Spoon(husband)
t0 = threading.Thread(target=husband.eatsWith, args=(spoon, wife))
t1 = threading.Thread(target=wife.eatsWith, args=(spoon, husband))
t0.start()
t1.start()
t0.join()
t1.join()
if __name__ == "__main__":
main()
I modify the answer of #jelbourn.
When one of them notices that the other is hungry, he(her) should release the spoon and wait another notify, so a livelock happens.
public class LiveLock {
static class Spoon {
Diner owner;
public String getOwnerName() {
return owner.getName();
}
public void setOwner(Diner diner) {
this.owner = diner;
}
public Spoon(Diner diner) {
this.owner = diner;
}
public void use() {
System.out.println(owner.getName() + " use this spoon and finish eat.");
}
}
static class Diner {
public Diner(boolean isHungry, String name) {
this.isHungry = isHungry;
this.name = name;
}
private boolean isHungry;
private String name;
public String getName() {
return name;
}
public void eatWith(Diner spouse, Spoon sharedSpoon) {
try {
synchronized (sharedSpoon) {
while (isHungry) {
while (!sharedSpoon.getOwnerName().equals(name)) {
sharedSpoon.wait();
//System.out.println("sharedSpoon belongs to" + sharedSpoon.getOwnerName())
}
if (spouse.isHungry) {
System.out.println(spouse.getName() + "is hungry,I should give it to him(her).");
sharedSpoon.setOwner(spouse);
sharedSpoon.notifyAll();
} else {
sharedSpoon.use();
sharedSpoon.setOwner(spouse);
isHungry = false;
}
Thread.sleep(500);
}
}
} catch (InterruptedException e) {
System.out.println(name + " is interrupted.");
}
}
}
public static void main(String[] args) {
final Diner husband = new Diner(true, "husband");
final Diner wife = new Diner(true, "wife");
final Spoon sharedSpoon = new Spoon(wife);
Thread h = new Thread() {
#Override
public void run() {
husband.eatWith(wife, sharedSpoon);
}
};
h.start();
Thread w = new Thread() {
#Override
public void run() {
wife.eatWith(husband, sharedSpoon);
}
};
w.start();
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
h.interrupt();
w.interrupt();
try {
h.join();
w.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
package concurrently.deadlock;
import static java.lang.System.out;
/* This is an example of livelock */
public class Dinner {
public static void main(String[] args) {
Spoon spoon = new Spoon();
Dish dish = new Dish();
new Thread(new Husband(spoon, dish)).start();
new Thread(new Wife(spoon, dish)).start();
}
}
class Spoon {
boolean isLocked;
}
class Dish {
boolean isLocked;
}
class Husband implements Runnable {
Spoon spoon;
Dish dish;
Husband(Spoon spoon, Dish dish) {
this.spoon = spoon;
this.dish = dish;
}
#Override
public void run() {
while (true) {
synchronized (spoon) {
spoon.isLocked = true;
out.println("husband get spoon");
try { Thread.sleep(2000); } catch (InterruptedException e) {}
if (dish.isLocked == true) {
spoon.isLocked = false; // give away spoon
out.println("husband pass away spoon");
continue;
}
synchronized (dish) {
dish.isLocked = true;
out.println("Husband is eating!");
}
dish.isLocked = false;
}
spoon.isLocked = false;
}
}
}
class Wife implements Runnable {
Spoon spoon;
Dish dish;
Wife(Spoon spoon, Dish dish) {
this.spoon = spoon;
this.dish = dish;
}
#Override
public void run() {
while (true) {
synchronized (dish) {
dish.isLocked = true;
out.println("wife get dish");
try { Thread.sleep(2000); } catch (InterruptedException e) {}
if (spoon.isLocked == true) {
dish.isLocked = false; // give away dish
out.println("wife pass away dish");
continue;
}
synchronized (spoon) {
spoon.isLocked = true;
out.println("Wife is eating!");
}
spoon.isLocked = false;
}
dish.isLocked = false;
}
}
}
Example:
Thread 1
top:
lock(L1);
if (try_lock(L2) != 0) {
unlock(L1);
goto top;
Thread 2
top:
lock(L2);
if (try_lock(L1) != 0) {
unlock(L2);
goto top;
The only difference is Thread 1 and Thread 2 try to acquire the locks in a different order. Livelock could happen as follows:
Thread 1 runs acquires L1, then a context switch occurs. Thread 2 runs acquires L2, then another context switch occurs. Thread 1 runs and cannot acquire L2, but before releasing L1 a context switch occurs. Thread 2 runs and cannot acquire L1, releases L2, and a context switch occurs. Thread 1 releases L1, and now we are basically back to the starting state, and in theory these steps could keep repeating forever.