Re: [PATCH] tools/memory-model: document the "one-time init" pattern

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Fri, Jul 17, 2020 at 06:47:50PM +0100, Matthew Wilcox wrote:
> 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!

I like it... :)

--D