forked from OSchip/llvm-project
81 lines
2.3 KiB
C
81 lines
2.3 KiB
C
#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;
|
|
|
|
}
|