[vlc-devel] [PATCH] posix/thread: use the monotonic clock for CV

[Rémi Denis-Courmont]

It depends what you mean by "initialized". This is C, not C++. The static initializers are constants. It's always safe to initialize an object to a constant value, so long as you don't use it.

What this does is no different that initializing a condition variable in a calloc'ed or memset data structure.

Le 24 février 2020 10:57:30 GMT+02:00, Thomas Guillem <thomas at gllm.fr> a écrit :
>
>
>On Sun, Feb 23, 2020, at 16:26, Rémi Denis-Courmont wrote:
>> This gets the earlier timed-wait behaviour back on NetBSD and other
>> platforms without futex.
>> ---
>>  src/posix/wait.c | 58
>++++++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 46 insertions(+), 12 deletions(-)
>> 
>> diff --git a/src/posix/wait.c b/src/posix/wait.c
>> index 5c146b61e2..26bce27448 100644
>> --- a/src/posix/wait.c
>> +++ b/src/posix/wait.c
>> @@ -32,6 +32,8 @@
>>  #include <sys/types.h>
>>  #include <pthread.h>
>>  
>> +static clockid_t vlc_clock_id = CLOCK_REALTIME;
>> +
>>  #define WAIT_BUCKET_INIT \
>>       { PTHREAD_MUTEX_INITIALIZER, PTHREAD_COND_INITIALIZER, 0 }
>>  #define WAIT_BUCKET_INIT_2 WAIT_BUCKET_INIT, WAIT_BUCKET_INIT
>> @@ -101,22 +103,29 @@ static int vlc_atomic_timedwait_timespec(void 
>> *addr, unsigned value,
>>      return ret;
>>  }
>>  
>> -int vlc_atomic_timedwait(void *addr, unsigned value, vlc_tick_t
>deadline)
>> +static void vlc_timespec_adjust(clockid_t cid, struct timespec
>*restrict ts)
>>  {
>> -    struct timespec ts;
>> -    vlc_tick_t delay = deadline - vlc_tick_now();
>> -    lldiv_t d = lldiv((delay >= 0) ? delay : 0, CLOCK_FREQ);
>> +    struct timespec now_from, now_to;
>> +    lldiv_t d;
>> +
>> +    if (vlc_clock_id == cid)
>> +        return;
>>  
>> -    /* TODO: use monotonic clock directly */
>> -    clock_gettime(CLOCK_REALTIME, &ts);
>> -    ts.tv_sec += d.quot;
>> -    ts.tv_nsec += NS_FROM_VLC_TICK(d.rem);
>> +    clock_gettime(cid, &now_from);
>> +    clock_gettime(vlc_clock_id, &now_to);
>>  
>> -    if (ts.tv_nsec >= 1000000000) {
>> -        ts.tv_sec++;
>> -        ts.tv_nsec -= 1000000000;
>> -    }
>> +    d = lldiv((ts->tv_sec - now_from.tv_sec + now_to.tv_sec) *
>1000000000LL
>> +              + ts->tv_nsec - now_from.tv_nsec - now_to.tv_nsec,
>1000000000LL);
>> +
>> +    ts->tv_sec = d.quot;
>> +    ts->tv_nsec = d.rem;
>> +}
>>  
>> +int vlc_atomic_timedwait(void *addr, unsigned value, vlc_tick_t
>deadline)
>> +{
>> +    struct timespec ts = timespec_from_vlc_tick(deadline);
>> +
>> +    vlc_timespec_adjust(CLOCK_MONOTONIC, &ts);
>>      return vlc_atomic_timedwait_timespec(addr, value, &ts);
>>  }
>>  
>> @@ -124,6 +133,7 @@ int vlc_atomic_timedwait_daytime(void *addr, 
>> unsigned value, time_t deadline)
>>  {
>>      struct timespec ts = { .tv_sec = deadline, .tv_nsec = 0 };
>>  
>> +    vlc_timespec_adjust(CLOCK_REALTIME, &ts);
>>      return vlc_atomic_timedwait_timespec(addr, value, &ts);
>>  }
>>  
>> @@ -141,3 +151,27 @@ void vlc_atomic_notify_all(void *addr)
>>          pthread_cond_broadcast(&bucket->wait);
>>      pthread_mutex_unlock(&bucket->lock);
>>  }
>> +
>> +#ifdef __ELF__
>> +__attribute__((constructor))
>> +static void vlc_atomic_clock_select(void)
>> +{
>> +    pthread_condattr_t attr;
>> +
>> +    pthread_condattr_init(&attr);
>> +    pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
>> +
>> +    for (size_t i = 0; i < ARRAY_SIZE(wait_buckets); i++)
>> +        pthread_cond_init(&wait_buckets[i].wait, &attr);
>
>So, the condition variables end up being initialized two times ?
>Here and by PTHREAD_COND_INITIALIZER.
>
>Are we sure it's safe ?
>
>> +
>> +    pthread_condattr_destroy(&attr);
>> +    vlc_clock_id = CLOCK_MONOTONIC;
>> +}
>> +
>> +__attribute__((destructor))
>> +static void vlc_atomic_clock_deselect(void)
>> +{
>> +    for (size_t i = 0; i < ARRAY_SIZE(wait_buckets); i++)
>> +        pthread_cond_destroy(&wait_buckets[i].wait);
>> +}
>> +#endif
>> -- 
>> 2.25.1
>> 
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