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

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Message-ID: <20130916084453.GA1222@redhat.com>
Date:	Mon, 16 Sep 2013 11:44:53 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	torvalds@...ux-foundation.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Why does test_bit() take a volatile addr?

On Mon, Sep 16, 2013 at 10:40:00AM +0200, Oliver Neukum wrote:
> On Mon, 2013-09-16 at 13:38 +0930, Rusty Russell wrote:
> > Predates git, does anyone remember the rationale?
> > 
> > ie:
> >         int test_bit(int nr, const volatile unsigned long *addr)
> > 
> > I noticed because gcc failed to elimiate some code in a patch I was
> > playing with.
> > 
> > I'm nervous about subtle bugs involved in ripping it out, even if noone
> > knows why.  Should I add __test_bit()?
> 
> It seems to me that if you do
> 
> b = *ptr & 0xf;
> c = b << 2;
> if (test_bit(1, ptr))
> 
> the compiler could optimize away the memory access on ptr without
> the volatile. We'd have to add a lot of mb().
> 
> 	Regards
> 		Oliver

What is this code supposed to do?
Any specific examples?

-- 
MST
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