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    SubjectRe: [PATCH] doc: add note on usleep_range range
    On Tue, Dec 13, 2016 at 04:27:32PM -0800, Joe Perches wrote:
    > a, On Tue, 2016-12-13 at 09:19 +0000, Nicholas Mc Guire wrote:
    > > On Tue, Dec 13, 2016 at 11:10:50AM +0200, Jani Nikula wrote:
    > > > On Tue, 13 Dec 2016, Nicholas Mc Guire <hofrat@osadl.org> wrote:
    > > > > useleep_range() with a delta of 0 makes no sense and only prevents the
    > > > > timer subsystem from optimizing interrupts. As any user of usleep_range()
    > > > > is in non-atomic context the timer jitter is in the range of 10s of
    > > > > microseconds anyway.
    > > > >
    > > > > This adds a note making it clear that a range of 0 is a bad idea.
    > > >
    > > > So I don't really have anything to do with the timer subsystem, I'm just
    > > > their "consumer", so take this with a grain of salt.
    > > >
    > > > Documentation is good, but I don't think this will be enough.
    > > >
    > > > I think the only thing that will work is to detect and complain about
    > > > things like this automatically. Some ideas:
    > > >
    > > > * WARN_ON(min == max) or WARN_ON_ONCE(min == max) in usleep_range()
    > > > might be drastic, but it would get the job done eventually.
    > > >
    > > > * If you want to avoid the runtime overhead (and complaints about the
    > > > backtraces), you could wrap usleep_range() in a macro that does
    > > > BUILD_BUG_ON(min == max) if the parameters are build time constants
    > > > (they usually are). But you'd have to fix all the problem cases first.
    > > >
    > > > * You could try (to persuade Julia or Dan) to come up with a
    > > > cocci/smatch check for usleep_range() calls where min == max, so we
    > > > could get bug reports for this. This probably works on expressions, so
    > > > this would catch also cases where the parameters aren't built timea,
    > > > constants.
    >
    > You could also add a macro for usleep_range like
    >
    > #define usleep_range(a, b) \
    > ({ \
    > if (__builtin_constant_p(a) && __builtin_constant_p(b)) { \
    > if (a == b) \
    > __compiletime_warning("Better to use usleep_range with different values"); \
    > else if (a > b) \
    > __compiletime_error("usleep_range uses smaller value first"); \
    > } \
    > usleep_range(a, b); \
    > })
    >

    thanks for that "template"

    > and add parentheses around the actual function
    > definition for usleep_range in kernel/time/timer.c
    > so the macro works and these messages get emitted
    > at compile-time.
    >
    while compiletime warnings are a way to go I think that an
    external tool is more effective than anoying eveyone during
    build - ideally this type of issue is filtered out in the
    subsystem trees or -next latest so getting it into a
    coccinelle spatch and into one of the CI seems the most
    resonable way to go. And as a side-effect tools external
    to the build process allow analysis into the history of the
    kernel development (like statistics on API usage and bug
    history).

    thx!
    hofrat

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