There is no class today. You don’t need to turn in anything for this exercise. Do try them for yourself though!
In this assignment we will explore how to implement a barrier using condition variables provided by the pthread library. A barrier is a point in an application at which all threads must wait until all other threads reach that point too. Condition variables are a sequence coordination technique similar to xv6’s sleep and wakeup.
Download barrier.c and compile it on your laptop or attu:
$ gcc -g -O2 -pthread barrier.c
$ ./a.out 2
Assertion failed: (i == t), function thread, file barrier.c, line 55.The 2 specifies the number of threads that synchronize on the barrier
(nthread in barrier.c). Each thread sits in a tight loop. In each
loop iteration a thread calls barrier() and then sleeps for some
random number of microseconds. The assert triggers, because one
thread leaves the barrier before the other thread has reached the
barrier. The desired behavior is that all threads should block until
nthreads have called barrier.
Your goal is to achieve the desired behavior. In addition to the
lock primitives that you have seen before, you will need the following
new pthread primitives (see man pthread for more detail):
pthread_cond_wait(&cond, &mutex); /* go to sleep on cond, releasing lock mutex */
pthread_cond_broadcast(&cond); /* wake up every thread sleeping on cond */pthread_cond_wait releases the mutex when called, and re-acquires
the mutex before returning.
We have given you barrier_init(). Your job is to implement barrier()
so that the panic won’t occur. We’ve defined struct barrier for
you; its fields are for your use.
There are two issues that complicate your task:
You have to deal with a succession of barrier calls, each of which
we’ll call a round. bstate.round records the current round. You
should increase bstate.round when each round starts.
You have to handle the case in which one thread races around the
loop before the others have exited the barrier. In particular, you
are re-using bstate.nthread from one round to the next. Make sure
that a thread that leaves the barrier and races around the loop
doesn’t increase bstate.nthread while a previous round is still
using it.
Test your code with one, two, and more than two threads.