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    SubjectRe: [patch] ipvs: force read of atomic_t in while loop
    Michael Buesch wrote:
    > On Thursday 09 August 2007 02:15:33 Andi Kleen wrote:
    >> On Wed, Aug 08, 2007 at 05:08:44PM -0400, Chris Snook wrote:
    >>> Heiko Carstens wrote:
    >>>> On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote:
    >>>>> From: Heiko Carstens <heiko.carstens@de.ibm.com>
    >>>>> Date: Wed, 8 Aug 2007 11:33:00 +0200
    >>>>>
    >>>>>> Just saw this while grepping for atomic_reads in a while loops.
    >>>>>> Maybe we should re-add the volatile to atomic_t. Not sure.
    >>>>> I think whatever the choice, it should be done consistently
    >>>>> on every architecture.
    >>>>>
    >>>>> It's just asking for trouble if your arch does it differently from
    >>>>> every other.
    >>>> Well..currently it's i386/x86_64 and s390 which have no volatile
    >>>> in atomic_t. And yes, of course I agree it should be consistent
    >>>> across all architectures. But it isn't.
    >>> Based on recent discussion, it's pretty clear that there's a lot of
    >>> confusion about this. A lot of people (myself included, until I thought
    >>> about it long and hard) will reasonably assume that calling
    >>> atomic_read() will actually read the value from memory. Leaving out the
    >>> volatile declaration seems like a pessimization to me. If you force
    >>> people to use barrier() everywhere they're working with atomic_t, it
    >>> will force re-reads of all the non-atomic data in use as well, which
    >>> will cause more memory fetches of things that generally don't need
    >>> barrier(). That and it's a bug waiting to happen.
    >>>
    >>> Andi -- your thoughts on the matter?
    >> I also think readding volatile makes sense. An alternative would be
    >> to stick an rmb() into atomic_read() -- that would also stop speculative reads.
    >> Disadvantage is that it clobbers all memory, not just the specific value.
    >>
    >> But you really have to complain to Linus (cc'ed). He came up
    >> with the volatile removale change iirc.
    >
    > Isn't it possible through some inline assembly trick
    > that only a certain variable has to be reloaded?
    > So we could define something like that:
    >
    > #define reload_var(x) __asm__ __volatile__ (whatever, x)
    >
    > I don't know inline assembly that much, but isn't it possible
    > with that to kind of "fake-touch" the variable, so the compiler
    > must reload it (and only it) to make sure it's up to date?

    We can do it in C, like this:

    -#define atomic_read(v) ((v)->counter)
    +#define atomic_read(v) (*(volatile int *)&(v)->counter)

    By casting it volatile at the precise piece of code where we want to guarantee a
    read from memory, there's little risk of the compiler getting creative in its
    interpretation of the code.

    Stay tuned for the patch set...

    -- Chris
    -
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