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    SubjectRe: [RFC][PATCH] rcu: Use typeof(p) instead of typeof(*p) *
    On Tue, 5 Oct 2021 21:06:36 +0200 (CEST)
    Jan Engelhardt <jengelh@inai.de> wrote:

    > On Tuesday 2021-10-05 20:40, Steven Rostedt wrote:
    > >>
    > >> >>>> typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p);
    > >>
    > >> #define static_cast(type, expr) ((struct { type x; }){(expr)}.x)
    > >> typeof(p) p1 = (typeof(p) __force)static_cast(void *, READ_ONCE(p));
    > >>
    > >> Let the name not fool you; it's absolutely _not_ the same as C++'s
    > >> static_cast, but still: it does emit a warning when you do pass an
    > >> integer, which is better than no warning at all in that case.
    > >>
    > >> *flies away*
    > >
    > >Are you suggesting I should continue this exercise ;-)
    >
    > “After all, why not?”
    >
    > typeof(p) p1 = (typeof(p) __force)READ_ONCE(p) +
    > BUILD_BUG_ON_EXPR(__builtin_classify_type(p) != 5);

    I may try it, because exposing the structure I want to hide, is pulling out
    a lot of other crap with it :-p

    struct trace_pid_list {
    raw_spinlock_t lock;
    struct irq_work refill_irqwork;
    union upper_chunk *upper[UPPER1_SIZE]; // 1 or 2K in size
    union upper_chunk *upper_list;
    union lower_chunk *lower_list;
    int free_upper_chunks;
    int free_lower_chunks;
    };

    I can still abstract out the unions, but this means I need to also pull out
    the define of "UPPER1_SIZE". Not to mention, I need to make sure irq_work
    and spin locks are defined.

    Another approach is to have it return:

    struct trace_pid_list {
    unsigned long ignore;
    };

    Rename the above struct trace_pid_list to struct trace_pid_internal.

    And internally have:

    union trace_pid_data {
    struct trace_pid_list external;
    struct trace_pid_internal internal;
    };

    Then use the internal version within the C file that modifies it, and just
    return a pointer to the external part.

    That should follow the "C standard".

    -- Steve

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