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

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Satyam Sharma wrote:
On Tue, 24 Jul 2007, Nick Piggin wrote:


Linus Torvalds wrote:


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.

Yeah that is probably what he meant.


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".

Because even with an allyesconfig, your compile isn't testing the
entire kernel. So given the relatively minor benefit of removing
the volatiles, I suppose we shouldn't risk slipping a bug in. If
you can make gcc throw an error in that case it would be a different
story.

--
SUSE Labs, Novell Inc.
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