linux-kernel - Re: [PATCH 4/8] i386: bitops: Kill volatile-casting of memory addresses

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Date:	Tue, 24 Jul 2007 19:49:03 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/8] i386: bitops: Kill volatile-casting of memory
	addresses


> The "const volatile" is so that you can pass an arbitrary pointer. The 
> only kind of abritraty pointer is "const volatile".
> 
> In other words, the "volatile" has nothing at all to do with whether the 
> memory is volatile or not (the same way "const" has nothing to do with it: 
> it's purely a C type *safety* issue, exactly the same way "const" is a 
> type safety issue.
> 
> A "const" on a pointer doesn't mean that the thing it points to cannot 
> change. When you pass a source pointer to "strlen()", it doesn't have to 
> be constant. But "strlen()" takes a "const" pointer, because it work son 
> constant pointers *too*.

However... What about that:

 - This "volatile" will allow to pass pointers to volatile data to the
bitops.

 - Most users of "volatile" in the kenrel (except maybe jiffies) are
bogus

 - Thus let's remove it -as a type safety thing- to catch more of those
stupid volatile that shouldn't be ? :-)

Besides, as Nick pointed out, it prevents some valid optimizations.

Cheers,
Ben.

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