Best practices for operating on a buffer in parallel?

[tclementdev]

@tera

var _array: [Element] = ...
...
pthread_mutex_lock(&mutex) // ***
let result = _array
pthread_mutex_unlock(&mutex)

just now I am having "Swift access race in App.Test.mutex.modify : __C._opaque_pthread_mutex_t at 0x7b1800023700" on *** line.

You cannot use the ampersand operator with pthread_mutex_lock() (and generally with low-level locks and atomics). This is a programming error that everybody makes. See this, this and this.