[vlc-devel] [PATCH 1/7] posix: add fallback vlc_atomic_* implementation

[Thomas Guillem]

On Sun, Feb 16, 2020, at 18:50, Rémi Denis-Courmont wrote:
> ---
>  src/Makefile.am  |   3 ++
>  src/posix/wait.c | 128 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 131 insertions(+)
>  create mode 100644 src/posix/wait.c
> 
> diff --git a/src/Makefile.am b/src/Makefile.am
> index f2adf61095..0ad47fc105 100644
> --- a/src/Makefile.am
> +++ b/src/Makefile.am
> @@ -482,6 +482,9 @@ libvlccore_la_SOURCES += \
>  	posix/plugin.c \
>  	posix/rand.c \
>  	posix/timer.c
> +if !HAVE_LINUX
> +libvlccore_la_SOURCES += posix/wait.c
> +endif
>  if !HAVE_ANDROID
>  libvlccore_la_SOURCES += posix/sort.c
>  if !HAVE_DARWIN
> diff --git a/src/posix/wait.c b/src/posix/wait.c
> new file mode 100644
> index 0000000000..60f9c7626f
> --- /dev/null
> +++ b/src/posix/wait.c
> @@ -0,0 +1,128 @@
> +/*****************************************************************************
> + * wait.c : pthread back-end for vlc_atomic_wait
> + *****************************************************************************
> + * Copyright (C) 2019-2020 Rémi Denis-Courmont
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms of the GNU Lesser General Public License as published by
> + * the Free Software Foundation; either version 2.1 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public License
> + * along with this program; if not, write to the Free Software Foundation,
> + * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
> + *****************************************************************************/
> +
> +#ifdef HAVE_CONFIG_H
> +# include "config.h"
> +#endif
> +
> +#include <vlc_common.h>
> +
> +#include <stdalign.h>
> +#include <stdatomic.h>
> +#include <errno.h>
> +#include <time.h>
> +
> +#include <sys/types.h>
> +#include <pthread.h>
> +
> +#define WAIT_BUCKET_INIT \
> +     { PTHREAD_MUTEX_INITIALIZER, PTHREAD_COND_INITIALIZER, 0 }
> +#define WAIT_BUCKET_INIT_2 WAIT_BUCKET_INIT, WAIT_BUCKET_INIT
> +#define WAIT_BUCKET_INIT_4 WAIT_BUCKET_INIT_2, WAIT_BUCKET_INIT_2
> +#define WAIT_BUCKET_INIT_8 WAIT_BUCKET_INIT_4, WAIT_BUCKET_INIT_4
> +#define WAIT_BUCKET_INIT_16 WAIT_BUCKET_INIT_8, WAIT_BUCKET_INIT_8
> +#define WAIT_BUCKET_INIT_32 WAIT_BUCKET_INIT_16, WAIT_BUCKET_INIT_16


If I understand correctly, this bucket is here to minimize the number of spurious wake, but it doesn't prevent it from happening, which is fine ?

> +
> +static struct wait_bucket
> +{
> +    pthread_mutex_t lock;
> +    pthread_cond_t wait;
> +    unsigned waiters;
> +} wait_buckets[32] = { WAIT_BUCKET_INIT_32 };
> +
> +static struct wait_bucket *wait_bucket_get(atomic_uint *addr)
> +{
> +    uintptr_t u = (uintptr_t)addr;
> +    size_t idx = (u / alignof (*addr)) % ARRAY_SIZE(wait_buckets);
> +
> +    return &wait_buckets[idx];
> +}
> +
> +static struct wait_bucket *wait_bucket_enter(atomic_uint *addr)
> +{
> +    struct wait_bucket *bucket = wait_bucket_get(addr);
> +
> +    pthread_mutex_lock(&bucket->lock);
> +    bucket->waiters++;
> +    return bucket;
> +}
> +
> +static void wait_bucket_leave(void *data)
> +{
> +    struct wait_bucket *bucket = data;
> +
> +    bucket->waiters--;
> +    pthread_mutex_unlock(&bucket->lock);
> +}
> +
> +void vlc_atomic_wait(void *addr, unsigned value)
> +{
> +    atomic_uint *futex = addr;
> +    struct wait_bucket *bucket = wait_bucket_enter(futex);
> +
> +    pthread_cleanup_push(wait_bucket_leave, bucket);
> +
> +    if (value == atomic_load_explicit(futex, memory_order_relaxed))
> +        pthread_cond_wait(&bucket->wait, &bucket->lock);
> +
> +    pthread_cleanup_pop(1);
> +}
> +
> +bool vlc_atomic_timedwait(void *addr, unsigned value, vlc_tick_t delay)
> +{
> +    atomic_uint *futex = addr;
> +    struct wait_bucket *bucket;
> +    struct timespec ts;
> +    lldiv_t d = lldiv(delay, CLOCK_FREQ);
> +    int ret = 0;
> +
> +    clock_gettime(CLOCK_MONOTONIC, &ts);
> +    ts.tv_sec += d.quot;
> +    ts.tv_nsec += NS_FROM_VLC_TICK(d.rem);
> +
> +    if (ts.tv_nsec >= 1000000000) {
> +        ts.tv_sec++;
> +        ts.tv_nsec -= 1000000000;
> +    }
> +
> +    bucket = wait_bucket_enter(futex);
> +    pthread_cleanup_push(wait_bucket_leave, bucket);
> +
> +    if (value == atomic_load_explicit(futex, memory_order_relaxed))
> +        ret = pthread_cond_timedwait(&bucket->wait, &bucket->lock, &ts);
> +
> +    pthread_cleanup_pop(1);
> +    return ret == 0;
> +}
> +
> +void vlc_atomic_notify_one(void *addr)
> +{
> +    vlc_atomic_notify_all(addr);
> +}
> +
> +void vlc_atomic_notify_all(void *addr)
> +{
> +    struct wait_bucket *bucket = wait_bucket_get(addr);
> +
> +    pthread_mutex_lock(&bucket->lock);
> +    if (bucket->waiters > 0)
> +        pthread_cond_broadcast(&bucket->wait);
> +    pthread_mutex_unlock(&bucket->lock);
> +}

OK for me, but it deserves a git message and/or few comments. It took me some time to understand what you wanted to do.

> -- 
> 2.25.0
> 
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