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| #include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
pthread_mutex_t contended_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t control_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t control_condition;
pthread_mutex_t thread_started_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t thread_started_condition;
// This function runs in a thread. The locking dance is to make sure that
// by the time the main thread reaches the pthread_join below, this thread
// has for sure acquired the contended_mutex. So then the call_me_to_get_lock
// function will block trying to get the mutex, and only succeed once it
// signals this thread, then lets it run to wake up from the cond_wait and
// release the mutex.
void *
lock_acquirer_1 (void *input)
{
pthread_mutex_lock (&contended_mutex);
// Grab this mutex, that will ensure that the main thread
// is in its cond_wait for it (since that's when it drops the mutex.
pthread_mutex_lock (&thread_started_mutex);
pthread_mutex_unlock(&thread_started_mutex);
// Now signal the main thread that it can continue, we have the contended lock
// so the call to call_me_to_get_lock won't make any progress till this
// thread gets a chance to run.
pthread_mutex_lock (&control_mutex);
pthread_cond_signal (&thread_started_condition);
pthread_cond_wait (&control_condition, &control_mutex);
pthread_mutex_unlock (&contended_mutex);
return NULL;
}
int
call_me_to_get_lock ()
{
pthread_cond_signal (&control_condition);
pthread_mutex_lock (&contended_mutex);
return 567;
}
int main ()
{
pthread_t thread_1;
pthread_cond_init (&control_condition, NULL);
pthread_cond_init (&thread_started_condition, NULL);
pthread_mutex_lock (&thread_started_mutex);
pthread_create (&thread_1, NULL, lock_acquirer_1, NULL);
pthread_cond_wait (&thread_started_condition, &thread_started_mutex);
pthread_mutex_lock (&control_mutex);
pthread_mutex_unlock (&control_mutex);
// Break here. At this point the other thread will have the contended_mutex,
// and be sitting in its cond_wait for the control condition. So there is
// no way that our by-hand calling of call_me_to_get_lock will proceed
// without running the first thread at least somewhat.
call_me_to_get_lock();
pthread_join (thread_1, NULL);
return 0;
}
|