Re: [PATCH 1/5] lib: introduce call_once()

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 11 Mar 2008 15:10:52 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> On Tuesday 11 March 2008 14:48, Andrew Morton wrote:
> > On Mon, 10 Mar 2008 23:57:05 +0900 Akinobu Mita <akinobu.mita@gmail.com> 
> wrote:
> > > call_once() is an utility function which has similar functionality of
> > > pthread_once().
> > >
> > > +/*
> > > + * call_once - call the initialization function only once
> > > + *
> > > + * @once_control: guarantee that the init_routine will be called only
> > > once + * @init_routine: initialization function
> > > + *
> > > + * The first call to call_once(), with a given once_control, shall call
> > > the + * init_routine with no arguments and return the value init_routine
> > > returned. + * If the init_routine returns zero which indicates the
> > > initialization + * succeeded, subsequent calls of call_once() with the
> > > same once_control shall + * not call the init_routine and return zero.
> > > + */
> > > +
> > > +static inline int call_once(struct once_control *once_control,
> > > +			    int (*init_rouine)(void))
> > > +{
> > > +	return likely(once_control->done) ? 0
> > > +			: call_once_slow(once_control, init_rouine);
> > > +}
> >
> > I don't believe that this shold be described in terms of an "init_routine".
> > This mechanism can be used for things other than initialisation routines.
> >
> > It is spelled "routine", not "rouine".
> >
> >
> > Would it not be simpler and more general to do:
> >
> > #define ONCE()						\
> > 	({						\
> > 		static long flag;			\
> > 							\
> > 		return !test_and_set_bit(0, flag);	\
> > 	})
> >
> > and then callers can do
> >
> > 	if (ONCE())
> > 		do_something();
> >
> > ?
> 
> Isn't this usually going to be buggy if you have concurrent access
> here? I'd prefer to keep synchronisation details in the caller and
> not have this call_once at all.

Well, I'm a bit dubious about the calue of all of this (althoug I didn't
review the callers).

But the above code is guaranteed race-free ;)