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

Akinobu Mita
2008-03-11T12:27:41Z

2008/3/11, 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();
> > >
> > > ?

This cannot be used directly in the conversions of patch 2 - 5.
Because the callers of call_once() in patch 2 - 5 need to wait for the
complition of "init_routine". Some blocking mechanism is needed.

> > 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).
>

I'll try to find another conversions to back up the necessity...