Inside an interrupt handler the code may not sleep, so a mutex is out of the question, yet the data still needs protecting. A spinlock is the answer: a waiting processor simply loops until the lock frees, which is cheap when the critical section is a few instructions and disastrous when it is long. In kernel Rust it has the same shape as the mutex, data inside, guard out, so the choice between them is a one-word edit driven purely by which context the lock is taken in. Taking a mutex from an interrupt handler, or holding a spinlock while allocating with the sleeping flag, is a classic machine freeze.
A spin lock and a mutex are two synchronization primitives used to enforce mutual exclusion over a critical section in concurrent programming, but they differ in how a thread waits when the resource …