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.
