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    SubjectRe: [PATCH] mask ADT: bitmap and bitop tweaks [1/22]
    At some point in the past, I wrote:
    >> akpm, this is needed for mainline.

    On Mon, Mar 29, 2004 at 05:46:37PM -0800, Paul Jackson wrote:
    > How urgent to you consider this fix (masking unused bits in the
    > arithmetic (single unsigned long word) cpumask implementation?
    > So far as I know, the only way to get high bits set with correct
    > invocations is by using cpus_complement(), which I don't see anyone
    > doing.
    > So I believe that this patch fixes latent bugs, not current bugs.

    False. The semantics are currently "don't care" and the ADT fails to
    ignore the upper bits in cpumask_arith.h. It's a bug in the ADT code.
    Whether callers experience ill effects is irrelevant.

    On Mon, Mar 29, 2004 at 05:46:37PM -0800, Paul Jackson wrote:
    > And it would be my preference (not surprisingly) to fix this in a way
    > that is consistent with my mask ADT proposal (avoid setting unused bits
    > on proper calls; don't filter on Boolean/scalar predicate evaluations):
    > +#if NR_CPUS % BITS_PER_LONG
    > +#define __CPU_VALID_MASK__ (~((1UL<< (NR_CPUS%BITS_PER_LONG) - 1))
    > +#else
    > +#define __CPU_VALID_MASK__ (~0UL)
    > +#endif
    > -#define cpus_complement(map) do { map = ~(map); } while (0)
    > +#define cpus_complement(map) \
    > + do { map = ~(map) & __CPU_VALID_MASK__; } while (0)
    > _instead_ of changing the several other macros to follow the
    > bitmap convention (let the unused bits remain dont-care, until
    > resolving a Boolean or scalar predicate).

    You're missing the changes needed for cpus_shift_left() and
    cpus_promote() to satisfy zeroed tail postconditions. IIRC the needed
    changes to cpus_shift_left() are also missing from your other patches
    in the bitmap code. You are also changing the invariants, which should
    be the substance of a patch different from any bugfix.

    -- wli
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