Atomic operations on pointers

[nordic-krch]

Is your feature request related to a problem? Please describe.

Currently atomic_t in atomic.h is set as int. On 32 bit platforms it is then ok to use it with pointers since they are the same size but not on 64 bit platforms. Code like:

int *p = (int *)0x12348985679890;
bool res = atomic_cas((atomic_t *)&p, (atomic_val_t)p, 0);

compiles with warning on qemu_x86_64 and sets to 0 only bottom 32 bits and returns true.

On the other hand using __atomic_compare_exchange_n directly works fine for pointers and integers.

I intend to use atomic_cas with pointers to locklessly control access to a module which has callback (if callback is set module is busy) and it cannot be done right now for 64 bit platforms. Something like:

if (atomic_cas(&obj->callback, NULL, callback) == false) {
  return -EBUSY;
}

Describe the solution you'd l`ike

Create atomic_ptr_t type and set of atomic_ptr_cas(), atomic_ptr_set(), atomic_ptr_get() and atomic_ptr_clear() functions.

[andrewboie]

We'll need this for #15138 as well

Where this gets annoying is the platforms that implement their own atomic ops using custom assembly.

I wonder if we could just change atomic_t to uintptr_t instead of int