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    SubjectRe: [PATCH 10/12] writeback: only allow one inflight and pending full flush
    On Fri, Sep 29, 2017 at 02:17:32AM +0200, Jens Axboe wrote:
    > On 09/28/2017 11:44 PM, Linus Torvalds wrote:
    > > On Thu, Sep 28, 2017 at 2:41 PM, Andrew Morton
    > > <akpm@linux-foundation.org> wrote:
    > >>
    > >> test_and_set_bit()?
    > >
    > > If there aren't any atomicity concerns (either because of higher-level
    > > locking, or because racing and having two people set the bit is fine),
    > > it can be better to do them separately if the test_bit() is the common
    > > case and you can avoid dirtying a cacheline that way.
    > >
    > > But yeah, if that is the case, it might be worth documenting, because
    > > test_and_set_bit() is the more obviously appropriate "there can be
    > > only one" model.
    >
    > It is documented though, but maybe not well enough...
    >
    > I've actually had to document/explain it enough times now, that it
    > might be worth making a general construct. Though it has to be
    > used carefully, so perhaps it's better contained as separate use
    > cases.

    test_and_test_and_set_bit()? It's an unusual name, so when either
    reading it or writing it, people are going to say "something unusual
    here", rather than "That Jens Axboe is such a n00b, he doesn't know how
    to use test_and_set_bit()". There are a few references out on the web
    to test-and-test-and-set already, so it's not entirely unique to Linux.

    Plus, some architectures might be able to optimise that, particularly
    those which are ll/sc based. It might be exactly the same as their
    test_and_set().

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