Mutexes are often used for critical resources to prevent multiple threads from accessing them at the same time. Sometimes, when locking mutexes, deadlock will happen when multiple threads hold each other's lock and the program consequently comes to a halt. There are four requirements for deadlock:
- Mutual Exclusion
- Hold and Wait
- No Preemption
- Circular Wait
Each deadlock requires all four conditions. Therefore, to prevent deadlock, prevent any one of the four conditions from being satisfied. This guideline recommends locking the mutexes in a predefined order to prevent circular wait.
Noncompliant Code Example
Based on runtime environment and the scheduler on the operating system, the following code will have different behaviors. However, with proper timing, the main() will deadlock when running thr1 and thr2 in which 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.
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
typedef struct {
int balance;
pthread_mutex_t balance_mutex;
} bank_account;
typedef struct {
bank_account *from;
bank_account *to;
int amount;
} deposit_thr_args;
/* return negative on error */
int create_bank_account(bank_account **ba, int initial_amount) {
int ret;
bank_account *nba = malloc(sizeof(bank_account));
if (nba == NULL) {
return -1;
}
nba->balance = initial_amount;
ret = pthread_mutex_init(&nba->balance_mutex, NULL);
if (ret)
exit(ret);
*ba = nba;
return 0;
}
void *deposit(void *ptr) {
deposit_thr_args *args = (deposit_thr_args *)ptr;
pthread_mutex_lock(&(args->from->balance_mutex));
/* not enough balance to transfer */
if (args->from->balance < args->amount) {
pthread_mutex_unlock(&(args->from->balance_mutex));
return NULL;
}
pthread_mutex_lock(&(args->to->balance_mutex));
args->from->balance -= args->amount;
args->to->balance += args->amount;
pthread_mutex_unlock(&(args->from->balance_mutex));
pthread_mutex_unlock(&(args->to->balance_mutex));
free(ptr);
return NULL;
}
int main() {
pthread_t thr1, thr2;
int err;
bank_account *ba1, *ba2;
err = create_bank_account(&ba1, 1000);
if (err < 0)
exit(err);
err = create_bank_account(&ba2, 1000);
if (err < 0)
exit(err);
deposit_thr_args *arg1 = malloc(sizeof(deposit_thr_args));
if (arg1 == NULL)
exit(-1);
deposit_thr_args *arg2 = malloc(sizeof(deposit_thr_args));
if (arg2 == NULL)
exit(-1);
arg1->from = ba1;
arg1->to = ba2;
arg1->amount = 100;
arg2->from = ba2;
arg2->to = ba1;
arg2->amount = 100;
/* perform the deposit */
err = pthread_create(&thr1, NULL, deposit, (void *)arg1);
if (err)
exit(err);
err = pthread_create(&thr2, NULL, deposit, (void *)arg2);
if (err)
exit(err);
pthread_exit(NULL);
return 0;
}
Compliant Solution
The solution to the deadlock problem is to lock in predefined order in the deposit() function. In the following example, each thread will lock based on bank_account's id defined in the struct initialization. This way circular wait problem is avoided and when one thread requires a lock will guarantee it will require the next lock.
typedef struct {
int balance;
pthread_mutex_t balance_mutex;
unsigned int id; /* read only and should never be changed */
} bank_account;
unsigned int global_id = 1;
/* return negative on error */
int create_bank_account(bank_account **ba, int initial_amount) {
int ret;
bank_account *nba = malloc(sizeof(bank_account));
if (nba == NULL) {
return -1;
}
nba->balance = initial_amount;
ret = pthread_mutex_init(&nba->balance_mutex, NULL);
if (ret)
exit(ret);
nba->id = global_id++;
*ba = nba;
return 0;
}
void *deposit(void *ptr) {
deposit_thr_args *args = (deposit_thr_args *)ptr;
if (args->from->id == args->to->id)
return NULL;
/* ensure proper ordering for locking */
if (args->from->id < args->to->id) {
pthread_mutex_lock(&(args->from->balance_mutex));
pthread_mutex_lock(&(args->to->balance_mutex));
} else {
pthread_mutex_lock(&(args->to->balance_mutex));
pthread_mutex_lock(&(args->from->balance_mutex));
}
/* not enough balance to transfer */
if (args->from->balance < args->amount) {
pthread_mutex_unlock(&(args->from->balance_mutex));
pthread_mutex_unlock(&(args->to->balance_mutex));
return NULL;
}
args->from->balance -= args->amount;
args->to->balance += args->amount;
pthread_mutex_unlock(&(args->from->balance_mutex));
pthread_mutex_unlock(&(args->to->balance_mutex));
free(ptr);
return NULL;
}
Risk Assessment
Deadlock causes multiple threads to become unable to progress and thus halts the executing program. This is a potential denial-of-service attack because the attacker can force deadlock situations. It is likely for deadlock to occur in multi-threaded programs that manage multiple shared resources.
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Level |
Priority |
|---|---|---|---|---|---|
POS43-C |
low |
probable |
medium |
L3 |
P3 |
Other Languages
This rule appears in the Java Secure Coding Standard as CON14-J. Avoid deadlock by requesting and releasing locks in the same order.
References
[pthread_mutex 
] pthread_mutex tutorial
[MITRE CWE:764 ] Multiple Locks of Critical Resources
[[Bryant 03]] Chapter 13, Concurrent Programming