Condition variable signalling issue

Question

I am in a soup. The idea may be bad but i do need a solution.

I have two condition variable, say A and B.
Threads 1, 2 and 3 are waiting on A. Thread 4 is waiting on B.
B will be pthread_cond-signal() by thread 2, that is thread 4 will be signaled to wake up by thread 2.

Now, I have another thread 5 which pthread_cond_broadcasts() on condition variable A. I need all threads 1, 2 and 3 to wake up before thread 4 wakes up. That is say if thread 2 wakes up and signals on B thread 4 may wake up before thread 3 does, which is not what i want.

Any pointers will be highly appreciated.

thanks

caf · Accepted Answer · 2011-08-04 07:27:49Z

2

You can solve this using condition variables. Instead of having Thread 4 wait just for the condition set by Thread 2, have it wait for a condition that is only set after all of Thread 1, Thread 2 and Thread 3 have done their thing:

pthread_mutex_lock(&thread4_lock);
while (!thread1_flag || !thread2_flag || !thread3_flag)
    pthread_cond_wait(&thread4_cond, &thread4_lock);
pthread_mutex_unlock(&thread4_lock);

/* Thread 4 can continue */

When Thread 1 has done its thing, it sets its part of the condition:

pthread_mutex_lock(&thread4_lock);
thread1_flag = 1;
pthread_cond_signal(&thread4_cond);
pthread_mutex_unlock(&thread4_lock);

...and likewise for Thread 2 and Thread 3. Thread 4 will only continue once all the flags have been set.

answered Aug 4, 2011 at 7:27

caf

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2 Comments

Adam Rosenfield Over a year ago

This will work, but don't forget that thread11_flag, thread2_flag, and thread3_flag should be declared volatile, and you also need a proper memory barrier on non-x86 systems to ensure that the memory updates are visible to other processors.

caf Over a year ago

@Adam Rosenfield: volatile and a barrier are not required, because those three flags are protected by the thread4_lock mutex as per the example.

Adam Rosenfield · Accepted Answer · 2011-08-04 03:20:13Z

1

Use a semaphore: have each of threads 1-3 post the semaphore, and have thread 4 wait on the semaphore 3 times instead of on a condition variable.

You'll want to use sem_init(3) or sem_open(3) to create the semaphore, sem_post(3) to post the semaphore, sem_wait(3) to wait on the semaphore, and then either sem_destroy(3) (if created with sem_init) or sem_close(3) and sem_unlink(3) (if created with sem_open) to destroy the semaphore.

answered Aug 4, 2011 at 3:20

Adam Rosenfield

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1 Comment

Lipika Deka Over a year ago

@Adam...great. Let me try out nd get back

Michael Dillon · Accepted Answer · 2011-08-04 05:08:37Z

0

Consider using ZeroMQ's in-process signalling for this. ZeroMQ implements a messaging model which can be used effectively to coordinate threads, as well as for inter process or network messaging.

For more info: http://api.zeromq.org/2-1-1:zmq-inproc

Here is an example of load-balancing between threads using ZeroMQ and Java.

answered Aug 4, 2011 at 5:08

Michael Dillon

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