CON53-CPP. Avoid deadlock by locking in a predefined order

Mutexes are used to prevent multiple threads from causing a data race by accessing shared resources at the same time. Sometimes, when locking mutexes, multiple threads hold each other's lock, and the program consequently deadlocks. Four conditions are required for deadlock to occur:

Mutual exclusion
Hold and wait
No preemption
Circular wait

Deadlock needs all four conditions, so preventing deadlock requires preventing any one of the four conditions. One simple solution is to lock the mutexes in a predefined order, which prevents circular wait.

Noncompliant Code Example

The behavior of this noncompliant code example depends on the runtime environment and the platform's scheduler. The program is susceptible to deadlock if thread thr1 attempts to lock ba2's mutex at the same time thread thr2 attempts to lock ba1's mutex in the deposit() function.

#include <mutex>
#include <thread>
 
class bank_account {
  int balance;
public:
  std::mutex balance_mutex;
  bank_account() = delete;
  bank_account(int initial_amount) : balance(initial_amount) {}
  int get_balance() { return balance; }
  void set_balance(int amount) { balance = amount; }
};
 
int deposit(bank_account *from, bank_account *to, int amount) {
  std::lock_guard<std::mutex> from_lock(from->balance_mutex);
 
  /* Not enough balance to transfer */
  if (from->get_balance() < amount) {
    return -1; /* Indicate error */
  }
  std::lock_guard<std::mutex> to_lock(to->balance_mutex);
 
  from->set_balance(from->get_balance() - amount);
  to->set_balance(to->get_balance() + amount);
 
  return 0;
}
 
int main(void) {
  bank_account *ba1 = new bank_account(1000);
  bank_account *ba2 = new bank_account(1000);
 
  /* Perform the deposits */
  std::thread thr1(deposit, ba1, ba2, 100);
  std::thread thr2(deposit, ba2, ba1, 100);
  thr1.join();
  thr2.join();
  return 0;
}

Compliant Solution

This compliant solution eliminates the circular wait condition by establishing a predefined order for locking in the deposit() function. Each thread will lock on the basis of the bank_account ID, which is set when the bank_account object is initialized.

#include <atomic>
#include <mutex>
#include <thread>
 
class bank_account {
  static std::atomic<unsigned int> global_id;
  const unsigned int id;
  int balance;
public:
  std::mutex balance_mutex;
  bank_account() = delete;
  bank_account(int initial_amount) : id(global_id++), balance(initial_amount) {}
  unsigned int get_id() { return id; }
  int get_balance() { return balance; }
  void set_balance(int amount) { balance = amount; }
};

std::atomic<unsigned int> bank_account::global_id(1);
 
int deposit(bank_account *from, bank_account *to, int amount) {
  int result = -1;
  std::mutex *first;
  std::mutex *second;
 
  if (from->get_id() == to->get_id()) {
    return -1; /* Indicate error */
  }
 
  /* Ensure proper ordering for locking */
  if (from->get_id() < to->get_id()) {
    first = &from->balance_mutex;
    second = &to->balance_mutex;
  } else {
    first = &to->balance_mutex;
    second = &from->balance_mutex;
  }
  std::lock_guard<std::mutex> first_lock(*first);
  std::lock_guard<std::mutex> second_lock(*second);
 
  /* Check for enough balance to transfer */
  if (from->get_balance() >= amount) {
    from->set_balance(from->get_balance() - amount);
    to->set_balance(to->get_balance() + amount);
    result = 0;
  }
 
  return result;
}

Risk Assessment

Deadlock prevents multiple threads from progressing, halting program execution. A denial-of-service attack is possible if the attacker can create the conditions for deadlock.

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
CON53-CPP	Low	Probable	Medium	P4	L3

Related Vulnerabilities

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

Automated Detection

Tool	Version	Checker	Description
CodeSonar	8.1p0	CONCURRENCY.LOCK.ORDER	Conflicting lock order
Coverity	6.5	DEADLOCK	Fully implemented
Parasoft C/C++test	9.5	BD-TRS-DLOCK	Fully implemented

Related Guidelines

CERT Oracle Secure Coding Standard for Java	LCK07-J. Avoid deadlock by requesting and releasing locks in the same order
MITRE CWE	CWE-764, Multiple Locks of a Critical Resource

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