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Mutexes are often used to prevent multiple threads from accessing critical resources at the same time. Sometimes, when locking mutexes, multiple threads hold each other's lock, and the program consequently deadlocks. There are four requirements for deadlock:

  • Mutual exclusion
  • Hold and wait
  • No preemption
  • Circular wait

Deadlock requires all four conditions, so to prevent deadlock, prevent any one of the four conditions. This guideline recommends locking the mutexes in a predefined order to prevent circular wait. This rule is a specific instance of CON35-C. Avoid deadlock by locking in predefined order using POSIX threads.

Noncompliant Code Example

This noncompliant code example has behavior that depends on the runtime environment and the platform's scheduler. However, with proper timing, the main() function will deadlock when running thr1 and thr2, where thr1 tries to lock ba2's mutex, while thr2 tries to lock on ba1's mutex in the deposit() function, and the program will not progress.

typedef struct {
  int balance;
  pthread_mutex_t balance_mutex;
} bank_account;

typedef struct {
  bank_account *from;
  bank_account *to;
  int amount;
} deposit_thr_args;

void create_bank_account(bank_account **ba, int initial_amount) {
  int result;
  bank_account *nba = malloc(sizeof(bank_account));
  if (nba == NULL) {
    /* Handle error */
  }

  nba->balance = initial_amount;
  result = pthread_mutex_init(&nba->balance_mutex, NULL);
  if (result) {
    /* Handle error */
  }

  *ba = nba;
}


void *deposit(void *ptr) {
  int result;
  deposit_thr_args *args = (deposit_thr_args *)ptr;

  if ((result = pthread_mutex_lock(&(args->from->balance_mutex))) != 0) {
    /* Handle error */
  }

  /* Not enough balance to transfer */
  if (args->from->balance < args->amount) {
    if ((result = pthread_mutex_unlock(&(args->from->balance_mutex))) != 0) {
      /* Handle error  */
    }
    return NULL;
  }

  if ((result = pthread_mutex_lock(&(args->to->balance_mutex))) != 0) {
    /* Handle error */
  }

  args->from->balance -= args->amount;
  args->to->balance += args->amount;

  if ((result = pthread_mutex_unlock(&(args->from->balance_mutex))) != 0) {
    /* Handle error */
  }
  if ((result = pthread_mutex_unlock(&(args->to->balance_mutex))) != 0) {
    /* Handle error */
  }

  free(ptr);
  return NULL;
}

int main(void) {

  pthread_t thr1, thr2;
  int result;

  bank_account *ba1;
  bank_account *ba2;
  create_bank_account(&ba1, 1000);
  create_bank_account(&ba2, 1000);

  deposit_thr_args *arg1 = malloc(sizeof(deposit_thr_args));
  if (arg1 == NULL) {
    /* Handle error */
  }
  deposit_thr_args *arg2 = malloc(sizeof(deposit_thr_args));
  if (arg2 == NULL) {
    /* Handle error */
  }

  arg1->from = ba1;
  arg1->to = ba2;
  arg1->amount = 100;

  arg2->from = ba2;
  arg2->to = ba1;
  arg2->amount = 100;

  /* Perform the deposits */
  if ((result = pthread_create(&thr1, NULL, deposit, (void *)arg1)) != 0) {
    /* Handle error */
  }
  if ((result = pthread_create(&thr2, NULL, deposit, (void *)arg2)) != 0) {
    /* Handle error */
  }

  pthread_exit(NULL);
  return 0;
}

Compliant Solution

The solution to the deadlock problem is to use a predefined order for the locks in the deposit() function. In this compliant solution, each thread will lock on the basis of the bank_account ID, defined in the struct initialization. This solution prevents the circular wait problem:

typedef struct {
  int balance;
  pthread_mutex_t balance_mutex;
  unsigned int id; /* Should never be changed after initialized */
} bank_account;

unsigned int global_id = 1;

void create_bank_account(bank_account **ba, int initial_amount) {
  int result;
  bank_account *nba = malloc(sizeof(bank_account));
  if (nba == NULL) {
    /* Handle error */
  }

  nba->balance = initial_amount;
  result = pthread_mutex_init(&nba->balance_mutex, NULL);
  if (result != 0) {
    /* Handle error */
  }

  nba->id = global_id++;
  *ba = nba;
}

void *deposit(void *ptr) {
  deposit_thr_args *args = (deposit_thr_args *)ptr;
  int result;

  if (args->from->id == args->to->id)
		return;

  /* Ensure proper ordering for locking */
  if (args->from->id < args->to->id) {
    if ((result = pthread_mutex_lock(&(args->from->balance_mutex))) != 0) {
      /* Handle error */
    }
    if ((result = pthread_mutex_lock(&(args->to->balance_mutex))) != 0) {
      /* Handle error */
    }
  } else {
    if ((result = pthread_mutex_lock(&(args->to->balance_mutex))) != 0) {
      /* Handle error */
    }
    if ((result = pthread_mutex_lock(&(args->from->balance_mutex))) != 0) {
      /* Handle error */
    }
  }

  /* Not enough balance to transfer */
  if (args->from->balance < args->amount) {
    if ((result = pthread_mutex_unlock(&(args->from->balance_mutex))) != 0) {
      /* Handle error */
    }
    if ((result = pthread_mutex_unlock(&(args->to->balance_mutex))) != 0) {
      /* Handle error */
    }
    return;
  }

  args->from->balance -= args->amount;
  args->to->balance += args->amount;

  if ((result = pthread_mutex_unlock(&(args->from->balance_mutex))) != 0) {
    /* Handle error */
  }
  if ((result = pthread_mutex_unlock(&(args->to->balance_mutex))) != 0) {
    /* Handle error */
  }

  free(ptr);
  return;
}

Risk Assessment

Deadlock prevents multiple threads from progressing, thus halting the executing program. A denial-of-service attack is possible because the attacker can force deadlock situations. Deadlock is likely to occur in multithreaded programs that manage multiple shared resources.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

POS51-C

Low

Probable

Medium

P4

L3

Automated Detection

ToolVersionCheckerDescription
CodeSonar
5.0p0
CONCURRENCY.LOCK.ORDERConflicting lock order
Klocwork
2018
CONC.DL
Parasoft C/C++test

10.4.2

CERT_C-POS51-a

Do not acquire locks in different order

Polyspace Bug Finder

R2018a

Deadlock

Call sequence to lock functions cause two tasks to block each other

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT CLCK07-J. Avoid deadlock by requesting and releasing locks in the same orderPrior to 2018-01-12: CERT: Unspecified Relationship

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-764 and POS51-C/POS35-C

Independent( CWE-764, POS51-C, POS35-C)

CWE-764 is about semaphores, or objects capable of being locked multiple times. Deadlock arises from multiple locks being acquired in a cyclic order, and generally does not arise from semaphores or recursive mutexes.

Bibliography

[Barney 2010]pthread_mutex tutorial
[Bryant 2003]Chapter 13, "Concurrent Programming"