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SubjectRe: Why does test_bit() take a volatile addr?
Hi Michael,

On Mon, 16 Sep 2013 10:26:03 +0300 "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> On Mon, Sep 16, 2013 at 04:53:44PM +1000, Stephen Rothwell wrote:
> >
> > On Mon, 16 Sep 2013 13:38:35 +0930 Rusty Russell <rusty@rustcorp.com.au> wrote:
> > >
> > > Predates git, does anyone remember the rationale?
> > >
> > > ie:
> > > int test_bit(int nr, const volatile unsigned long *addr)
> >
> > Because we sometimes pass volatile pointers to it and gcc will complain
> > if you pass a volatile to a non volatile (I think).
>
> Where are these? I did git grep -W test_bit and looked for volatile,
> couldn't find anything.

OK, so it was a bit of a guess. Have you really checked the type of
every address passed to every call of test_bit()?

Second guess: we wanted to make the test_bit access volatile (as opposed
to the datatypes of the objects being tested) so that things like

while (testbit(bit, addr)) {
do_very_little();
}

don't get over optimised (since we are operating in a very threaded
environment that the compiler not might expect).

--
Cheers,
Stephen Rothwell sfr@canb.auug.org.au
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