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


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