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Noncompliant Code Example (pthread_cond_signal())
The following noncompliant code example consists on a given number of threads (5) that should execute one after another according to the step level that is assigned to them when created (serialized processing). The time variable holds the current step level and is incremented as soon as the respective thread finishes its processing. Finally, another thread is signaled so that the next step can be executed.
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#include <stdio.h> #include <stdlib.h> #include <pthread.h> #define NTHREADS 105 pthread_mutex_t mutex; pthread_cond_t cond; int main(int argc, char** argv) { int i; int result; pthread_attr_t attr; pthread_t threads[NTHREADS]; int step[NTHREADS]; if ((result = pthread_mutex_init(&mutex, NULL)) != 0) { /* Handle error condition */ } if ((result = pthread_cond_init(&cond, NULL)) != 0) { /* Handle error condition */ } if ((result = pthread_attr_init(&attr)) != 0) { /* Handle error condition */ } if ((result = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE)) != 0) { /* Handle error condition */ } /* Create threads */ for (i = 0; i < NTHREADS; i++) { step[i] = i; if ((result = pthread_create(&threads[i], &attr, run_step, (void *)step[i])) != 0) { /* Handle error condition */ } } /* Wait for all threads to complete */ for (i = NTHREADS-1; i >= 0; i--) { if ((result = pthread_join(threads[i], NULL)) != 0) { /* Handle error condition */ } } if ((result = pthread_mutex_destroy(&mutex)) != 0) { /* Handle error condition */ } if ((result = pthread_cond_destroy(&cond)) != 0) { /* Handle error condition */ } if ((result = pthread_attr_destroy(&attr)) != 0) { /* Handle error condition */ } pthread_exit(NULL); } void *run_step(void *t) { static int time = 0; int step = (int)t; int result; if ((result = pthread_mutex_lock(&mutex)) != 0) { /* Handle error condition */ } printf("Thread %d has the lock\n", step); while (time != step) { printf("Thread %d is sleeping...\n", step); if ((result = pthread_cond_wait(&cond, &mutex)) != 0) { /* Handle error condition */ } printf("Thread %d woke up\n", step); } /* Do processing... */ printf("Thread %d is processing...\n", step); time++; /* Signal a waiting task */ if ((result = pthread_cond_signal(&cond)) != 0) { /* Handle error condition */ } printf("Thread %d is exiting...\n", step); if ((result = pthread_mutex_unlock(&mutex)) != 0) { /* Handle error condition */ } pthread_exit(NULL); } |
In the above code, each thread has its own predicate because each requires time to have a different value before proceeding.
Having into consideration that upon the signal operation pthread_cond_signal() any of the waiting threads can wake up and that if by chance it is not the one with the next step value, that one will wait again pthread_cond_wait(), thus resulting in a deadlock situation because no more signal operations will occur.
Therefore, this noncompliant code example violates the liveness property.
Compliant Solution (using pthread_cond_broadcast())
This compliant solution uses the pthread_cond_broadcast() method to signal all waiting threads instead of a single "random" one.
Only the run_step() thread code from the noncompliant code example is modified, as follows:
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void *run_step(void *t) { static int time = 0; int step = (int)t; int result; if ((result = pthread_mutex_lock(&mutex)) != 0) { /* Handle error condition */ } printf("Thread %d has the lock\n", step); while (time != step) { printf("Thread %d is sleeping...\n", step); if ((result = pthread_cond_wait(&cond, &mutex)) != 0) { /* Handle error condition */ } printf("Thread %d woke up\n", step); } /* Do processing... */ printf("Thread %d is processing...\n", step); time++; /* Signal ALL waiting tasks */ if ((result = pthread_cond_signalbroadcast(&cond)) != 0) { /* Handle error condition */ } printf("Thread %d is exiting...\n", step); if ((result = pthread_mutex_unlock(&mutex)) != 0) { /* Handle error condition */ } pthread_exit(NULL); } |
The fact that all tasks will be waken up solves the problem because all tasks end up executing its predicate test, and therefore one will find it to be true and continue the execution until the end.
Compliant Solution (using pthread_cond_signal() but with a unique condition variable per thread)
One way to solve the signal issue is to use a unique condition variable for each thread (maintaining a single mutex associated with it). In this case, the signal operation (pthread_cond_signal()) will wake up the only thread waiting on it.
Note that the predicate of the signaled thread must be true, otherwise a deadlock may occur anyway).
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#include <stdio.h> #include <stdlib.h> #include <pthread.h> #define NTHREADS 5 pthread_mutex_t mutex; pthread_cond_t cond[NTHREADS]; int main(int argc, char** argv) { int i; int result; pthread_attr_t attr; pthread_t threads[NTHREADS]; int step[NTHREADS]; if ((result = pthread_mutex_init(&mutex, NULL)) != 0) { /* Handle error condition */ } for (i = 0; i< NTHREADS; i++) { if ((result = pthread_cond_init(&cond[i], NULL)) != 0) { /* Handle error condition */ } } if ((result = pthread_attr_init(&attr)) != 0) { /* Handle error condition */ } if ((result = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE)) != 0) { /* Handle error condition */ } /* Create threads */ for (i = 0; i<NTHREADSi < NTHREADS; i++) { step[i] = i; if ((result = pthread_create(&threads[i], &attr, run_step, (void *)step[i])) != 0) { /* Handle error condition */ } } /* Wait for all threads to complete */ for (i = NTHREADS-1; i >= 0; i--) { if ((result = pthread_join(threads[i], NULL)) != 0) { /* Handle error condition */ } } if ((result = pthread_mutex_destroy(&mutex)) != 0) { /* Handle error condition */ } for (i = 0; i < NTHREADS; i++) { if ((result = pthread_cond_destroy(&cond[i])) != 0) { /* Handle error condition */ } } if ((result = pthread_attr_destroy(&attr)) != 0) { /* Handle error condition */ } pthread_exit(NULL); } |
Compliant Solution (pthread_cond_broadcast())
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This compliant solution uses the {{pthread_cond_broadcast()}} method to signal all waiting threads. Before {{await()}} returns, the current thread reacquires the lock associated with this condition. When the thread returns, it is guaranteed to hold this lock \[[API 2006|AA. Bibliography#API 06]\] The thread that is ready can perform its task, while all the threads whose condition predicates are false resume waiting. |
Only the run() method from the noncompliant code example is modified, as follows:
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void *run_step(void *t) {
static int time = 0;
int step = (int)t;
int result;
if ((result = pthread_mutex_lock(&mutex)) != 0) {
/* Handle error condition */
}
printf("Thread %d has the lock\n", step);
while (time != step) {
printf("Thread %d is sleeping...\n", step);
if ((result = pthread_cond_wait(&cond[step], &mutex)) != 0) {
/* Handle error condition */
}
printf("Thread %d woke up\n", step);
}
/* Do processing... */
printf("Thread %d is processing...\n", step);
time++;
/* Signal next step thread */
if ((step + 1) < NTHREADS) {
if ((result = pthread_cond_signal(&cond[step+1])) != 0) {
/* Handle error condition */
}
}
printf("Thread %d is exiting...\n", step);
if ((result = pthread_mutex_unlock(&mutex)) != 0) {
/* Handle error condition */
}
pthread_exit(NULL);
}
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In the above code each thread has associated a unique condition variable which is signaled when that particular thread needs to be waken up. This solution turns to be more efficient because only the desired thread will be waken up.
Compliant Solution (pthread_cond_signal() but with one condition variable per thread)
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Risk Assessment
Signal a single thread instead of all waiting threads can pose a threat to the liveness property of the system.
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