On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote:
> +If that doesn't apply, you'll have to implement one-time init yourself.
> +
> +The simplest implementation just uses a mutex and an 'inited' flag.
> +This implementation should be used where feasible:
I think some syntactic sugar should make it feasible for normal people
to implement the most efficient version of this just like they use locks.
> +For the single-pointer case, a further optimized implementation
> +eliminates the mutex and instead uses compare-and-exchange:
> +
> + static struct foo *foo;
> +
> + int init_foo_if_needed(void)
> + {
> + struct foo *p;
> +
> + /* pairs with successful cmpxchg_release() below */
> + if (smp_load_acquire(&foo))
> + return 0;
> +
> + p = alloc_foo();
> + if (!p)
> + return -ENOMEM;
> +
> + /* on success, pairs with smp_load_acquire() above and below */
> + if (cmpxchg_release(&foo, NULL, p) != NULL) {
Why do we have cmpxchg_release() anyway? Under what circumstances is
cmpxchg() useful _without_ having release semantics?
> + free_foo(p);
> + /* pairs with successful cmpxchg_release() above */
> + smp_load_acquire(&foo);
> + }
> + return 0;
> + }
How about something like this ...
once.h:
static struct init_once_pointer {
void *p;
};
static inline void *once_get(struct init_once_pointer *oncep)
{ ... }
static inline bool once_store(struct init_once_pointer *oncep, void *p)
{ ... }
--- foo.c ---
struct foo *get_foo(gfp_t gfp)
{
static struct init_once_pointer my_foo;
struct foo *foop;
foop = once_get(&my_foo);
if (foop)
return foop;
foop = alloc_foo(gfp);
if (!once_store(&my_foo, foop)) {
free_foo(foop);
foop = once_get(&my_foo);
}
return foop;
}
Any kernel programmer should be able to handle that pattern. And no mutex!
