On Mon, Jul 27, 2020 at 04:28:27PM +0100, Matthew Wilcox wrote:
> On Mon, Jul 27, 2020 at 11:17:46AM -0400, Alan Stern wrote:
> > Given a type "T", an object x of type pointer-to-T, and a function
> > "func" that takes various arguments and returns a pointer-to-T, the
> > accepted API for calling func once would be to create once_func() as
> > follows:
> >
> > T *once_func(T **ppt, args...)
> > {
> > static DEFINE_MUTEX(mut);
> > T *p;
> >
> > p = smp_load_acquire(ppt); /* Mild optimization */
> > if (p)
> > return p;
> >
> > mutex_lock(mut);
> > p = smp_load_acquire(ppt);
> > if (!p) {
> > p = func(args...);
> > if (!IS_ERR_OR_NULL(p))
> > smp_store_release(ppt, p);
> > }
> > mutex_unlock(mut);
> > return p;
> > }
> >
> > Users then would have to call once_func(&x, args...) and check the
> > result. Different x objects would constitute different "once"
> > domains.
> [...]
> > In fact, the only drawback I can think of is that because this relies
> > on a single mutex for all the different possible x's, it might lead to
> > locking conflicts (if func had to call once_func() recursively, for
> > example). In most reasonable situations such conflicts would not
> > arise.
>
> Another drawback for this approach relative to my get_foo() approach
> upthread is that, because we don't have compiler support, there's no
> enforcement that accesses to 'x' go through once_func(). My approach
> wraps accesses in a deliberately-opaque struct so you have to write
> some really ugly code to get at the raw value, and it's just easier to
> call get_foo().
Something like that could be included in once_func too. It's relatively
tangential to the main point I was making, which was to settle on an
overall API and discuss how it should be described in recipes.txt.
Alan Stern
