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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 condition can degrade the performance of a system or cause a deadlock to occur.

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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 blocks until data arrives on the socket. While it is blocked, other threads that are waiting for the lock will are 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

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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 The blocking behavior consequently affects only the thread that called the blocking function.

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This compliant solution performs the recv() call with the parameter o_nonblock, which causes the call to fail if there are no messages are 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 */
  }
}

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POS52-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 guideline CON35-C. Avoid deadlock by locking in predefined order.

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The CERT Oracle Secure Coding Standard for Java: LCK09-J. Do not perform operations that can block while holding a lock

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Sources

[Barney 2010] POSIX Threads Programming
[Open Group] pthread_cancel(), recv()

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