Blocking operation while holding lock

Task performs lengthy operation while holding a lock

Description

This defect occurs when a task (thread) performs a potentially lengthy operation while holding a lock.

The checker considers calls to these functions as potentially lengthy:

• Functions that access a network such as recv

• System call functions such as fork, pipe and system

• Functions for I/O operations such as getchar and scanf

• File handling functions such as fopen, remove and lstat

• Directory manipulation functions such as mkdir and rmdir

The checker automatically detects certain primitives that hold and release a lock, for instance, pthread_mutex_lock and pthread_mutex_unlock. For the full list of primitives that are automatically detected, see the documentation of Polyspace^® Bug Finder™ or Polyspace Bug Finder Server™.

Risk

If a thread performs a lengthy operation when holding a lock, other threads that use the lock have to wait for the lock to be available. As a result, system performance can slow down or deadlocks can occur.

Fix

Perform the blocking operation before holding the lock or after releasing the lock.

Some functions detected by this checker can be called in a way that does not make them potentially lengthy. For instance, the function recv can be called with the parameter O_NONBLOCK which causes the call to fail if no message is available. When called with this parameter, recv does not wait for a message to become available.

Examples

Network I/O Operations with recv While Holding Lock

#include <pthread.h>
#include <sys/socket.h>

pthread_mutexattr_t attr;
pthread_mutex_t mutex;
 
void thread_foo(void *ptr) {
  unsigned int 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(unsigned int), 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(
      &attr, PTHREAD_MUTEX_ERRORCHECK)) != 0) {
    /* Handle Error */
  }
 
  if ((result = pthread_mutex_init(&mutex, &attr)) != 0) {
    /* Handle Error */
  }
 
  if (pthread_create(&thread, NULL,(void*(*)(void*))& thread_foo, NULL) != 0) {
    /* Handle Error */
  }
 
  /* ... */
 
  pthread_join(thread, NULL);
 
  if ((result = pthread_mutex_destroy(&mutex)) != 0) {
    /* Handle Error */
  }
 
  return 0;
}

In this example, in each thread created with pthread_create, the function thread_foo performs a network I/O operation with recv after acquiring a lock with pthread_mutex_lock. Other threads using the same lock variable mutex have to wait for the operation to complete and the lock to become available.

Correction — Perform Blocking Operation Before Acquiring Lock

One possible correction is to call recv before acquiring the lock.

#include <pthread.h>
#include <sys/socket.h>

pthread_mutexattr_t attr;
pthread_mutex_t mutex;
 
void thread_foo(void *ptr) {
  unsigned int num;
  int result;
  int sock;
 
  /* sock is a connected TCP socket */
  if ((result = recv(sock, (void *)&num, sizeof(unsigned int), 0)) < 0) {
    /* Handle Error */
  }
  
  if ((result = pthread_mutex_lock(&mutex)) != 0) {
    /* Handle Error */
  }
 
    /* ... */
 
  if ((result = pthread_mutex_unlock(&mutex)) != 0) {
    /* Handle Error */
  }
}
 
int main() {
  pthread_t thread;
  int result;
 
  if ((result = pthread_mutexattr_settype(
      &attr, PTHREAD_MUTEX_ERRORCHECK)) != 0) {
    /* Handle Error */
  }
 
  if ((result = pthread_mutex_init(&mutex, &attr)) != 0) {
    /* Handle Error */
  }
 
  if (pthread_create(&thread, NULL,(void*(*)(void*))& thread_foo, NULL) != 0) {
    /* Handle Error */
  }
 
  /* ... */
 
  pthread_join(thread, NULL);
 
  if ((result = pthread_mutex_destroy(&mutex)) != 0) {
    /* Handle Error */
  }
 
  return 0;
}

Result Information

Group: Concurrency

Language: C | C++

Default: Off

Command-Line Syntax: BLOCKING_WHILE_LOCKED

Impact: Low

CWE ID: 667