pthread_mutex_t should not be locked when already locked, or unlocked when already unlocked.
Why is this an issue?
Mutexes are synchronization primitives that allow to manage concurrency. This is the most fundamental building block for creating
safe concurrent applications. By using a mutex, one can ensure that a block of code is executed by a single thread concurrently.
Data structures are designed to maintain their invariants between member-function calls. If a data structure is accessed concurrently, and one of the
accesses is a write operation, then it has a data race. Having data races is undefined behavior.
Adversaries actively exploit data races to take over systems, but data races are also a common source of data corruption
in concurrent applications resulting in dormant and hard-to-find bugs.
To prevent data races, the shared resource (usually memory) must be protected by obtaining mutual access to the data during both reading
and writing. Such mutual exclusion is generally achieved by using a mutex, which is frequently referred to as a lock.
A mutex has two states: released - which is the initial state, or acquired. These two states are frequently called
unlocked and locked as well.
To effectively protect the shared resource from concurrent accesses, all such accesses should be guarded by the same mutex. They need to
lock the mutex to gain safe exclusive access to the resource and unlock it after they are done mutating or reading
it.
You can abstract away the concurrent threads sharing the mutex and think of it as owned by the current thread. It never spontaneously changes
between acquired and released.
In this view, these are the possible transitions when calling lock or unlock on a mutex in a given state:
- released +
lock() ⇒ acquired
- acquired +
unlock() ⇒ released
- acquired +
lock() ⇒ deadlock
- released +
unlock() ⇒ undefined behavior
When a thread locks a mutex, another thread trying to acquire the same mutex will be blocked and have to wait for the first
thread to release it. This waiting period can take some time. If a thread attempts to lock a mutex it has already acquired, it will
deadlock because it would need to release it to lock it again.
What is the potential impact?
Locking an acquired mutex leads to a deadlock, as a mutex can only be obtained once. Unlocking a released mutex is
undefined behavior. Removing synchronization can cause data races, leading to data corruption, which adversaries might
leverage to take over the system.
Exceptions
There are recursive mutexes that can be acquired multiple times by the same thread, given that just as many times we also
release the mutex. They are rare in practice and usually signal design problems in the code. Thus we assume
pthread_mutex_t refers to non-recursive mutexes.
How to fix it
Never lock an acquired mutex. Lock the mutexes in a strict order that is followed throughout the project. Unlock operations should be done
the same way but in reversed order.
Code examples
Noncompliant code example
#include <pthread.h>
pthread_mutex_t m1 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t m2 = PTHREAD_MUTEX_INITIALIZER;
void locks() {
pthread_mutex_lock(&m1);
pthread_mutex_lock(&m1); // Noncompliant: 'm1' is already acquired
}
void unlocks() {
pthread_mutex_unlock(&m1);
pthread_mutex_unlock(&m1); // Noncompliant: 'm1' is already released
}
Compliant solution
#include <pthread.h>
pthread_mutex_t m1 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t m2 = PTHREAD_MUTEX_INITIALIZER;
void locks() {
pthread_mutex_lock(&m1);
pthread_mutex_lock(&m2); // Compliant: we acquired both 'm1' and 'm2' for the first time.
}
void unlocks() {
pthread_mutex_unlock(&m2);
pthread_mutex_unlock(&m1); // Compliant: we released both 'm1' and 'm2' for the first time.
}
Pitfalls
Calling arbitrary functions while holding a lock should be avoided, as the function might also want to lock the resource we already acquired,
causing a deadlock. One particular example of such functions is callbacks.
#include <pthread.h>
pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
void handle_callback(void (*callback)(void)) {
pthread_mutex_lock(&m);
callback(); // If tries to lock mutex 'm', then we have a deadlock.
pthread_mutex_unlock(&m);
}
Going the extra mile
The section of code for which the mutex is acquired is called the critical section. Inside the critical
section, we are the only ones with access to the shared resource. Thus we are free to mutate or read it without considering what other threads
are doing concurrently. However, having large critical sections can prevent other threads from making progress if they want to also
acquire the same mutex and access the shared resource. Consequently, critical sections are supposed to be as small as
possible.
#include <pthread.h>
#include <stdbool.h>
int input;
int result;
bool isFib;
// Guards both 'num1' and 'num2'.
pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
int fibonacci(int n);
int factorial(int n);
void locks(bool calcFib, int n) {
// Do the calculations without taking the lock.
int res;
if (calcFib) {
res = fibonacci(n);
} else {
res = factorial(n);
}
pthread_mutex_lock(&m);
// Critical section starts
input = n;
result = res;
isFib = calcFib;
// Critical section ends.
pthread_mutex_unlock(&m);
}
Resources
Standards
Related rules
- S5487 enforces proper initialization and destruction of
pthread mutexes.
- S5489 enforces unlocking held
pthread mutexes in reverse order.
Documentation
