[FFmpeg-devel] [PATCH 24/42] avcodec/refstruct: Allow to share pools

[Andreas Rheinhardt]

Anton Khirnov:
> Quoting Andreas Rheinhardt (2023-09-19 21:57:16)
>> To do this, make FFRefStructPool itself refcounted according
>> to the RefStruct API.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
>> ---
>>  libavcodec/refstruct.c | 29 ++++++++++++++++-------------
>>  libavcodec/refstruct.h |  5 ++++-
>>  2 files changed, 20 insertions(+), 14 deletions(-)
>>
>> diff --git a/libavcodec/refstruct.c b/libavcodec/refstruct.c
>> index f8d040874d..2108ff8163 100644
>> --- a/libavcodec/refstruct.c
>> +++ b/libavcodec/refstruct.c
>> @@ -187,7 +187,7 @@ static void pool_free(FFRefStructPool *pool)
>>      pthread_mutex_destroy(&pool->mutex);
>>      if (pool->free_cb)
>>          pool->free_cb(pool->opaque);
>> -    av_free(pool);
>> +    av_free(get_refcount(pool));
>>  }
>>  
>>  static void pool_free_entry(FFRefStructPool *pool, RefCount *ref)
>> @@ -278,13 +278,17 @@ void *ff_refstruct_pool_get(FFRefStructPool *pool)
>>      return ret;
>>  }
>>  
>> -void ff_refstruct_pool_uninit(FFRefStructPool **poolp)
>> +static void pool_unref(void *ref)
>>  {
>> -    FFRefStructPool *pool = *poolp;
>> -    RefCount *entry;
>> +    FFRefStructPool *pool = get_userdata(ref);
>> +    if (atomic_fetch_sub_explicit(&pool->refcount, 1, memory_order_acq_rel) == 1)
> 
> Is there a reason you cannot fold pool->refcount into the pool's
> containing RefStruct?
> 

If I simply incremented the pool's refcount for every entry currently in
use by users, then the entries would only be freed when the last entry
has been returned and all the references to the pool unreferenced.

In fact, when I did this, I pondered two things: Shall I make
ff_refstruct_pool_uninit() free all the currently available buffers and
then unreference the caller's reference or shall I just make it a
wrapper to ff_refstruct_unref() to decrement the pool's refcount? The
latter is very simple and I did it; the former could be advantageous in
particular in case of frame-threading in case the dimensions change. (In
this scenario, no user will ever create new entries after the first user
unreferences a pool.)

- Andreas