CON31-C. Do not destroy a mutex while it is locked

Mutexes are used to protect shared data structures being concurrently accessed. If a mutex is destroyed while a thread is blocked waiting for that mutex, critical sections (shared data that would otherwise be protected from data races) are no longer protected.

The C Standard, 7.26.4.1, paragraph 2 [ISO/IEC 9899:2011], states:

The mtx_destroy function releases any resources used by the mutex pointed to by mtx. No threads can be blocked waiting for the mutex pointed to by mtx.

This statement implies that destroying a mutex while a thread is waiting on it is undefined behavior.

Noncompliant Code Example

This noncompliant code example creates several threads that each invoke the do_work() function, passing a unique number as an ID. The do_work() function initializes the lock mutex if the argument is 0 and destroys the mutex if the argument is max_threads - 1. In all other cases, the do_work() function provides normal processing. Each thread, except the final cleanup thread, increments the atomic completed variable when it is finished.

Unfortunately, this code contains several race conditions, allowing the mutex to be destroyed before it is unlocked. Additionally, there is no guarantee that lock will be initialized before it is passed to mtx_lock(). Each of these behaviors is undefined.

#include <stdatomic.h>
#include <stddef.h>
#include <threads.h>
 
mtx_t lock;
/* Atomic so multiple threads can modify safely */
atomic_int completed = ATOMIC_VAR_INIT(0);
enum { max_threads = 5 }; 

int do_work(void *arg) {
  int *i = (int *)arg;

  if (*i == 0) { /* Creation thread */
    if (thrd_success != mtx_init(&lock, mtx_plain)) {
      /* Handle error */
    }
    atomic_store(&completed, 1);
  } else if (*i < max_threads - 1) { /* Worker thread */
    if (thrd_success != mtx_lock(&lock)) {
      /* Handle error */
    }
    /* Access data protected by the lock */
    atomic_fetch_add(&completed, 1);
    if (thrd_success != mtx_unlock(&lock)) {
      /* Handle error */
    }
  } else { /* Destruction thread */
    mtx_destroy(&lock);
  }
  return 0;
}
 
int main(void) {
  thrd_t threads[max_threads];
  
  for (size_t i = 0; i < max_threads; i++) {
    if (thrd_success != thrd_create(&threads[i], do_work, &i)) {
      /* Handle error */
    }
  }
  for (size_t i = 0; i < max_threads; i++) {
    if (thrd_success != thrd_join(threads[i], 0)) {
      /* Handle error */
    }
  }
  return 0;
}

Compliant Solution

This compliant solution eliminates the race conditions by initializing the mutex in main() before creating the threads and by destroying the mutex in main() after joining the threads:

#include <stdatomic.h>
#include <stddef.h>
#include <threads.h>
 
mtx_t lock;
/* Atomic so multiple threads can increment safely */
atomic_int completed = ATOMIC_VAR_INIT(0);
enum { max_threads = 5 }; 

int do_work(void *dummy) {
  if (thrd_success != mtx_lock(&lock)) {
    /* Handle error */
  }
  /* Access data protected by the lock */
  atomic_fetch_add(&completed, 1);
  if (thrd_success != mtx_unlock(&lock)) {
    /* Handle error */
  }

  return 0;
}

int main(void) {
  thrd_t threads[max_threads];
  
  if (thrd_success != mtx_init(&lock, mtx_plain)) {
    /* Handle error */
  }
  for (size_t i = 0; i < max_threads; i++) {
    if (thrd_success != thrd_create(&threads[i], do_work, NULL)) {
      /* Handle error */
    }
  }
  for (size_t i = 0; i < max_threads; i++) {
    if (thrd_success != thrd_join(threads[i], 0)) {
      /* Handle error */
    }
  }

  mtx_destroy(&lock);
  return 0;
}

Risk Assessment

Destroying a mutex while it is locked may result in invalid control flow and data corruption.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
CON31-C	Medium	Probable	High	P4	L3

Automated Detection

Tool	Version	Checker	Description
Fortify SCA	5.0		Can detect violations of this rule with CERT C Rule Pack
Parasoft C/C++test	9.5	BD-RES-FREE, BD-RES-INVFREE

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

MITRE CWE

CWE-667, Improper Locking

Bibliography

[ISO/IEC 9899:2011] 7.26.4.1, "The mtx_destroy Function"

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