On Fri, Jul 17, 2020 at 10:28:18PM -0700, Eric Biggers wrote:
> /**
> * INIT_ONCE() - do one-time initialization
> * @done: pointer to a 'bool' flag that tracks whether initialization has been
> * done yet or not. Must be false by default.
> * @mutex: pointer to a mutex to use to synchronize executions of @init_func
> * @init_func: the one-time initialization function
> * @...: additional arguments to pass to @init_func (optional)
> *
> * This is a more general version of DO_ONCE_BLOCKING() which supports
> * non-static data by allowing the user to specify their own 'done' flag and
> * mutex.
> *
> * Return: 0 on success (done or already done), or a negative errno value
> * returned by @init_func.
It might be worth pointing out explicitly that init_func can be called
multiple times, if it returns an error.
> */
> #define INIT_ONCE(done, mutex, init_func, ...) \
> ({ \
> int err = 0; \
> \
> if (!smp_load_acquire(done)) { \
> mutex_lock(mutex); \
> if (!*(done)) { \
> err = init_func(__VA_ARGS__); \
> if (!err) \
> smp_store_release((done), true); \
> } \
> mutex_unlock(mutex); \
> } \
> err; \
> })
If this macro is invoked in multiple places for the same object (which
is not unlikely), there is a distinct risk that people will supply
different mutexes or done variables for the invocations.
IMO a better approach would be to have a macro which, given a variable
name v, generates an actual init_once_v() function. Then code wanting
to use v would call init_once_v() first, with no danger of inconsistent
usage. You can fill in the details...
Alan Stern
