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

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Message-ID: <1379320800.15916.15.camel@linux-fkkt.site>
Date:	Mon, 16 Sep 2013 10:40:00 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	torvalds@...ux-foundation.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Why does test_bit() take a volatile addr?

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


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