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    SubjectRe: [RFC] Event counters [1/3]: Basic counter functionality
    On Mon, 2006-01-02 at 19:40 -0200, Marcelo Tosatti wrote:

    > Nick,
    > The following patch:
    > - Moves the lightweight "inc/dec" versions of mod_page_state variants
    > to three underscores, making those the default for locations where enough
    > locks are held.

    I guess I was hoping to try to keep it simple, and just have two
    variants, the __ version would require the caller to do the locking.
    In cases like eg. allocstall, they should happen infrequently enough
    that the extra complexity is probably not worth worrying about.

    I don't think I commented about the preempt race though (and requirement
    to have preempt off from process context), which obviously can be a
    problem as you say (though I think things are currently safe?).

    > - Make the two-underscore version disable and enable preemption, which
    > is required to avoid preempt-related races which can result in missed
    > updates.
    > - Extends the lightweight version usage in page reclaim,
    > pte allocation, and a few other codepaths.

    I guess nr_dirty looks OK in the places it can be put under tree_lock.

    nr_page_table_pages is OK because ptl should be held to prevent preempt.

    pgrotated and pgactivate should be good because of lru_lock.

    Thanks for going through these!

    SUSE Labs, Novell Inc.

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