[vlc-devel] [PATCH v3 01/10] core: add atomic refcounter helper

[Rémi Denis-Courmont]

Le tiistaina 3. heinäkuuta 2018, 9.47.52 EEST Romain Vimont a écrit :
> Implement an atomic refcounter with a weak but correct (1) memory order,
> and expose a simple API.
> 
> (1) See for example "Using weakly ordered C++ atomics correctly" by Hans
>     Boehm at CppCon 2016 (the refcounting part also applies to C11):
>     <https://www.youtube.com/watch?v=M15UKpNlpeM&t=45m15s>
> ---
>  include/vlc_atomic.h | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/include/vlc_atomic.h b/include/vlc_atomic.h
> index 868918fe8e6..1b09ff61937 100644
> --- a/include/vlc_atomic.h
> +++ b/include/vlc_atomic.h
> @@ -30,7 +30,9 @@
>   * Atomic operations do not require locking, but they are not very
> powerful. */
> 
> +# include <assert.h>
>  # include <stdatomic.h>
> +# include <vlc_common.h>
> 
>  typedef atomic_uint_least32_t vlc_atomic_float;
> 
> @@ -57,4 +59,28 @@ static inline void
> vlc_atomic_store_float(vlc_atomic_float *atom, float f) atomic_store(atom,
> u.i);
>  }
> 
> +typedef atomic_uint vlc_atomic_rc_t;

Wrap in struct to avoid accidental layering violation.

> +
> +/** Init the RC to 1 */
> +static inline void vlc_atomic_rc_init(vlc_atomic_rc_t *var)
> +{
> +    atomic_init(var, 1);
> +}
> +
> +/** Increment the RC */
> +static inline void vlc_atomic_rc_inc(vlc_atomic_rc_t *var)
> +{
> +    unsigned prev = atomic_fetch_add_explicit(var, 1,
> memory_order_relaxed); +    vlc_assert(prev);
> +    VLC_UNUSED(prev);
> +}
> +
> +/** Decrement the RC and return true if it reaches 0 */
> +static inline bool vlc_atomic_rc_dec(vlc_atomic_rc_t *var)
> +{
> +    unsigned prev = atomic_fetch_sub_explicit(var, 1,
> memory_order_acq_rel); +    vlc_assert(prev);
> +    return prev == 1;
> +}
> +
>  #endif


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