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SubjectRe: [PATCH 4/8] i386: bitops: Kill volatile-casting of memory addresses
On Tue, 24 Jul 2007, Nick Piggin wrote:

> Linus Torvalds wrote:
> >
> > On Mon, 23 Jul 2007, Satyam Sharma wrote:
> >
> >
> > > [4/8] i386: bitops: Kill volatile-casting of memory addresses
> >
> >
> > This is wrong.
> >
> > 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*.
> >
> > Same deal here.
> >
> > Admittedly this may be mostly historic, but regardless - the "volatiles" are
> > right.
> >
> > Using volatile on *data* is generally considered incorrect and bad taste,
> > but using it in situations like this potentially makes sense.
> >
> > Of course, if we remove all "volatiles" in data in the kernel (with the
> > possible exception of "jiffies"), we can then remove them from function
> > declarations too, but it should be done in that order.
>
> Well, regardless, it still forces the function to treat the pointer
> target as volatile, won't it? It definitely prevents valid optimisations
> that would be useful for me in mm/page_alloc.c where page flags are
> being set up or torn down or checked with non-atomic bitops.

Yes, and yes. But I think what he meant there is that we'd need to
audit the kernel for all users of set_bit and friends and see if callers
actually pass in any _data_ that _is_ volatile. So we have to kill them
there first, and then in the function declarations here. I think I'll put
that on my long-term todo list, but see below.

> Anyway by type safety, do you mean it will stop the compiler from
> warning if a pointer to a volatile is passed to the bitop?

The compiler would start warning for all those cases (passing in
a pointer to volatile data, when the bitops have lost the volatile
casting from their function declarations), actually. Something like
"passing argument discards qualifiers from pointer type" ... but
considering I didn't see *any* of those warnings after these patches,
I'm confused as to what exactly Linus meant here ... and what exactly
do we need to do "kill the volatiles".

> If so, then
> why don't we just kill all the volatiles out of here and fix any
> warnings that comeup? I doubt there would be many, and of those, some
> might show up real synchronisation problems.

Yes, but see above.

> OK, not the i386 functions as much because they are all in asm anwyay,
> but in general (btw. why does i386 or any architecture define their own
> non-atomic bitops? If the version in asm-generic/bitops/non-atomic.h
> is not good enough then surely it is a bug in gcc or that file?)



Satyam
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