CON36-C. Do not perform operations that can block while holding a lock

If a lock is being held and an operation that can block is performed, any other thread that needs to acquire that lock may also block. This can degrade the performance of a system or cause a deadlock to occur.

Blocking calls include, but are not limited to: network, file, and console I/O.

Noncompliant Code Example

This noncompliant code example demonstrates an occurrence of a blocking call that waits to receive data on a socket while a mutex is locked. The recv() call will block until data arrives on the socket. While it is blocked, other threads that are waiting for the lock will also be blocked.

Although this example is specific to network I/O, the recv() call could be replaced with any blocking call and the same behavior would occur.

pthread_mutexattr_t attr;
pthread_mutex_t mutex;

void thread_foo(void *ptr) {
  uint32_t num;
  int result;
  int sock;

  /* sock is a connected TCP socket */

  if ((result = pthread_mutex_lock(&mutex)) != 0) {
    /* Handle Error */
  }

  if ((result = recv(sock, (void *)&num, sizeof(uint32_t), 0)) < 0) {
    /* Handle Error */
  }

  /* ... */

  if ((result = pthread_mutex_unlock(&mutex)) != 0) {
    /* Handle Error */
  }
}

int main() {
  pthread_t thread;
  int result;

  if ((result = pthread_mutexattr_settype(
      &mutex, PTHREAD_MUTEX_ERRORCHECK)) != 0) {
    /* Handle Error */
  }

  if ((result = pthread_mutex_init(&mutex, &attr)) != 0) {
    /* Handle Error */
  }

  if (pthread_create(&thread, NULL,(void *)& thread_foo, NULL) != 0) {
    /* Handle Error */
  }

  /* ... */

  pthread_join(thread, NULL);

  if ((result = pthread_mutex_destroy(&mutex)) != 0) {
    /* Handle Error */
  }

  return 0;
}

Compliant Solution (Block while not locked)

This compliant solution performs the recv() call when the lock has not been acquired. This causes the blocking behavior to only affect the thread that called the blocking function.

void thread_foo(void *ptr) {
  uint32_t num;
  int result;
  int sock;

  /* sock is a connected TCP socket */

  if ((result = recv(sock, (void *)&num, sizeof(uint32_t), 0)) < 0) {
    /* Handle Error */
  }

  if ((result = pthread_mutex_lock(&mutex)) != 0) {
    /* Handle Error */
  }

  /* ... */

  if ((result = pthread_mutex_unlock(&mutex)) != 0) {
    /* Handle Error */
  }
}

Compliant Solution (Use a non-blocking call)

This compliant solution performs the recv() call with the parameter o_nonblock, which causes the call to fail if there are no messages available on the socket.

void thread_foo(void *ptr) {
  uint32_t num;
  int result;

  /* sock is a connected TCP socket */

  if ((result = recv(sock, (void *)&num, sizeof(uint32_t), O_NONBLOCK)) < 0) {
    /* Handle Error */
  }

  if ((result = pthread_mutex_lock(&mutex)) != 0) {
    /* Handle Error */
  }

  /* ... */

  if ((result = pthread_mutex_unlock(&mutex)) != 0) {
    /* Handle Error */
  }
}

Exceptions

CON36-EX1: A thread may block while holding one or more locks and waiting to acquire another lock. When acquiring multiple locks, the order of locking must avoid deadlock, as specified in CON35-C. Avoid deadlock by locking in predefined order.