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