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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/5] lib: introduce call_once()
Date: Tue, 11 Mar 2008 15:10:52 +1100
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Cc: Akinobu Mita <akinobu.mita@gmail.com>, linux-kernel@vger.kernel.org
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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.