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

Rémi Denis-Courmont remi at remlab.net
Mon Feb 17 14:35:40 CET 2020


I want the APIs to work on all platforms, and this adds a generic POSIX implementation. Exactly what it says already. I don't know what else you want to explain.

Le 17 février 2020 14:39:08 GMT+02:00, Thomas Guillem <thomas at gllm.fr> a écrit :
>
>
>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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