| #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; |
| |
| } |