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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)
    Mathieu Desnoyers wrote:
    > * Nicholas Miell (nmiell@comcast.net) wrote:
    >> On Wed, 2010-01-13 at 13:24 -0500, Mathieu Desnoyers wrote:
    >>> * Nicholas Miell (nmiell@comcast.net) wrote:
    >>>
    >>>> The whole point of compat and incompat flags is that it allows new
    >>>> applications to run on old kernels and either work or fail as
    >>>> appropriate, depending on whether the new features they're using must be
    >>>> implemented or can be silently ignored.
    >>> I see. Thanks for the explanation. Then the expedited flag should
    >>> clearly be part of the mandatory flags.
    >>>
    >>> Can you point me to other system calls that are doing this ?
    >>>
    >>> Thanks,
    >>>
    >>> Mathieu
    >> Not off the top of my head, but I did steal the idea from the ext2/3/4
    >> disk format.
    >
    > Sounds a bit over-engineered to me for system calls, but who knows if we
    > eventually have to extend sys_membarrier(). This involves that, right
    > now, I'd have to add a header to include/linux to define these flags.
    > Also, "int expedited" is a bit clearer, but less flexible, than "int
    > flags". Anyone else have comments about this ?
    >

    It doesn't bother me that you have to do extra work to add the flag
    definitions to a header file. :-)

    As I understand it, the proposal is to have the option to extend the ABI
    based on as yet undefined flag bits. This doesn't seem like a bad thing.

    The runtime overhead of testing a single bit vs. non-zero in the
    parameter shouldn't be an issue.

    David Daney


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