> This doesn't get initialised anywhere.
>
> Presumably you're relying on a memset somewhere. That might work on all
> architectures, AFAIK. But in theory it's wrong. If, for example, the
> architecture implements atomic_t via a spinlock-plus-integer, and that
> spinlock's unlocked state is not all-bits-zero, we're dead.
>
> So we should initialise it with
>
> foo->num_waiting = ATOMIC_INIT(0);
>
Is it correct to use a structure initializer this way?
> nb: it is not correct to initialise an atomic_t with
>
> atomic_set(a, 0);
>
> because in the above theoretical case case where the arch uses a spinlock
> in the atomic_t, that spinlock doesn't get initialised. I bet we've got code
> in there which does this.
According to Documentation/atomic_ops.txt, this is the correct usage
of atomic_set():
| The first operations to implement for atomic_t's are the
| initializers and plain reads.
|
| #define ATOMIC_INIT(i) { (i) }
| #define atomic_set(v, i) ((v)->counter = (i))
|
| The first macro is used in definitions, such as:
|
| static atomic_t my_counter = ATOMIC_INIT(1);
|
| The second interface can be used at runtime, as in:
|
| struct foo { atomic_t counter; };
| ...
|
| struct foo *k;
|
| k = kmalloc(sizeof(*k), GFP_KERNEL);
| if (!k)
| return -ENOMEM;
| atomic_set(&k->counter, 0);
So in fact atomic_set() is an initializer, and should be named
atomic_init() accordingly. Is atomic_set() ever used as an atomic
operation rather than an initializer?
Miklos
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