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    Subject[PATCH v1 3/5] seqlock: seqcount_t: Implement all read APIs as statement expressions
    Date
    The sequence counters read APIs are implemented as CPP macros, so they
    can take either seqcount_t or any of the seqcount_LOCKTYPE_t variants.
    Such macros then get *directly* transformed to internal C functions that
    only take plain seqcount_t.

    Further commits need access to seqcount_LOCKTYPE_t inside of the actual
    read APIs code. Thus transform all of the seqcount read APIs to pure GCC
    statement expressions instead.

    This will not break type-safety: all of the transformed APIs resolve to
    a _Generic() selection that does not have a "default" case.

    This will also not affect the transformed APIs readability: previously
    added kernel-doc above all of seqlock.h functions makes the expectations
    quite clear for call-site developers.

    Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
    ---
    include/linux/seqlock.h | 98 ++++++++++++++++++++---------------------
    1 file changed, 49 insertions(+), 49 deletions(-)

    diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
    index 5470e9cd52ce..d114a9f4e9d9 100644
    --- a/include/linux/seqlock.h
    +++ b/include/linux/seqlock.h
    @@ -182,6 +182,12 @@ __seqcount_##locktype##_ptr(seqcount_##locktype##_t *s) \
    return &s->seqcount; \
    } \
    \
    +static __always_inline unsigned \
    +__seqcount_##locktype##_sequence(const seqcount_##locktype##_t *s) \
    +{ \
    + return READ_ONCE(s->seqcount.sequence); \
    +} \
    + \
    static __always_inline bool \
    __seqcount_##locktype##_preemptible(const seqcount_##locktype##_t *s) \
    { \
    @@ -203,6 +209,11 @@ static inline seqcount_t *__seqcount_t_ptr(seqcount_t *s)
    return s;
    }

    +static inline unsigned __seqcount_t_sequence(const seqcount_t *s)
    +{
    + return READ_ONCE(s->sequence);
    +}
    +
    static inline bool __seqcount_t_preemptible(const seqcount_t *s)
    {
    return false;
    @@ -248,6 +259,7 @@ SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base)
    __seqprop_case((s), ww_mutex, prop))

    #define __seqcount_ptr(s) __seqprop(s, ptr)
    +#define __seqcount_sequence(s) __seqprop(s, sequence)
    #define __seqcount_lock_preemptible(s) __seqprop(s, preemptible)
    #define __seqcount_assert_lock_held(s) __seqprop(s, assert)

    @@ -266,21 +278,19 @@ SEQCOUNT_LOCKTYPE(ww_mutex, struct ww_mutex, true, &s->lock->base)
    * Return: count to be passed to read_seqcount_retry()
    */
    #define __read_seqcount_begin(s) \
    - __read_seqcount_t_begin(__seqcount_ptr(s))
    -
    -static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
    -{
    - unsigned ret;
    -
    -repeat:
    - ret = READ_ONCE(s->sequence);
    - if (unlikely(ret & 1)) {
    - cpu_relax();
    - goto repeat;
    - }
    - kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
    - return ret;
    -}
    +({ \
    + unsigned seq; \
    + \
    + do { \
    + seq = __seqcount_sequence(s); \
    + if (likely(! (seq & 1))) \
    + break; \
    + cpu_relax(); \
    + } while (true); \
    + \
    + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
    + seq; \
    +})

    /**
    * raw_read_seqcount_begin() - begin a seqcount_t read section w/o lockdep
    @@ -289,14 +299,12 @@ static inline unsigned __read_seqcount_t_begin(const seqcount_t *s)
    * Return: count to be passed to read_seqcount_retry()
    */
    #define raw_read_seqcount_begin(s) \
    - raw_read_seqcount_t_begin(__seqcount_ptr(s))
    -
    -static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
    -{
    - unsigned ret = __read_seqcount_t_begin(s);
    - smp_rmb();
    - return ret;
    -}
    +({ \
    + unsigned seq = __read_seqcount_begin(s); \
    + \
    + smp_rmb(); \
    + seq; \
    +})

    /**
    * read_seqcount_begin() - begin a seqcount_t read critical section
    @@ -305,13 +313,10 @@ static inline unsigned raw_read_seqcount_t_begin(const seqcount_t *s)
    * Return: count to be passed to read_seqcount_retry()
    */
    #define read_seqcount_begin(s) \
    - read_seqcount_t_begin(__seqcount_ptr(s))
    -
    -static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
    -{
    - seqcount_lockdep_reader_access(s);
    - return raw_read_seqcount_t_begin(s);
    -}
    +({ \
    + seqcount_lockdep_reader_access(__seqcount_ptr(s)); \
    + raw_read_seqcount_begin(s); \
    +})

    /**
    * raw_read_seqcount() - read the raw seqcount_t counter value
    @@ -325,15 +330,13 @@ static inline unsigned read_seqcount_t_begin(const seqcount_t *s)
    * Return: count to be passed to read_seqcount_retry()
    */
    #define raw_read_seqcount(s) \
    - raw_read_seqcount_t(__seqcount_ptr(s))
    -
    -static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
    -{
    - unsigned ret = READ_ONCE(s->sequence);
    - smp_rmb();
    - kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
    - return ret;
    -}
    +({ \
    + unsigned seq = __seqcount_sequence(s); \
    + \
    + smp_rmb(); \
    + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
    + seq; \
    +})

    /**
    * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
    @@ -353,16 +356,13 @@ static inline unsigned raw_read_seqcount_t(const seqcount_t *s)
    * Return: count to be passed to read_seqcount_retry()
    */
    #define raw_seqcount_begin(s) \
    - raw_seqcount_t_begin(__seqcount_ptr(s))
    -
    -static inline unsigned raw_seqcount_t_begin(const seqcount_t *s)
    -{
    - /*
    - * If the counter is odd, let read_seqcount_retry() fail
    - * by decrementing the counter.
    - */
    - return raw_read_seqcount_t(s) & ~1;
    -}
    +({ \
    + /* \
    + * If the counter is odd, let read_seqcount_retry() fail \
    + * by decrementing the counter. \
    + */ \
    + raw_read_seqcount(s) & ~1; \
    +})

    /**
    * __read_seqcount_retry() - end a seqcount_t read section w/o barrier
    --
    2.28.0
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      Last update: 2020-08-28 03:08    [W:2.368 / U:0.024 seconds]
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