linux-kernel - Re: Why does test_bit() take a volatile addr?

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Message-Id: <20130916180231.1570f7c5cebab599cef5b4bb@canb.auug.org.au>
Date:	Mon, 16 Sep 2013 18:02:31 +1000
From:	Stephen Rothwell <sfr@...b.auug.org.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Why does test_bit() take a volatile addr?

Hi Michael,

On Mon, 16 Sep 2013 10:26:03 +0300 "Michael S. Tsirkin" <mst@...hat.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@...tcorp.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@...b.auug.org.au

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