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    SubjectRe: KCSAN: data-race in __alloc_file / __alloc_file
    On Fri, Nov 8, 2019 at 12:30 PM Linus Torvalds
    <torvalds@linux-foundation.org> wrote:
    >
    > On Fri, Nov 8, 2019 at 9:56 AM Eric Dumazet <edumazet@google.com> wrote:
    > >
    > > BTW, I would love an efficient ADD_ONCE(variable, value)
    > >
    > > Using WRITE_ONCE(variable, variable + value) is not good, since it can
    > > not use the optimized instructions operating directly on memory.
    >
    > So I'm having a hard time seeing how this could possibly ever be valid.
    >
    > Is this a "writer is locked, readers are unlocked" case or something?

    per cpu SNMP counters mostly, with no IRQ safety requirements.

    Note that this could be implemented using local{64}_add() on arches like x86_64,
    while others might have to fallback to WRITE_ONCE(variable, variable + add)

    >
    > Because we don't really have any sane way to do that any more
    > efficiently, unless we'd have to add new architecture-specific
    > functions for it (like we do have fo the percpu ops).
    >
    > Anyway, if you have a really hot case you care about, maybe you could
    > convince the gcc people to just add it as a peephole optimization?
    > Right now, gcc ends up doing some strange things with volatiles, and
    > basically disables a lot of stuff over them. But with a test-case,
    > maybe you can convince somebody that certain optimizations are still
    > fine. A "read+add+write" really does the exact same accesses as an
    > add-to-memory instruction, but gcc has some logic to disable that
    > instruction fusion.
    >
    > Linus

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