[vlc-devel] [RFC] [NOT FOR MERGE] objres: add mutex support

[Steve Lhomme]

On 2020-01-29 8:49, Steve Lhomme wrote:
> On 2020-01-29 8:37, Thomas Guillem wrote:
>> Hello,
>>
>> I would prefer to avoid using a pointer to access a mutex.
>>
>> We will end up with modules using vlc_mutex_lock(sys->lock) and 
>> vlc_mutex_lock(&sys->lock).
>> I think it will add more confusions.
> 
> How about vlc_obj_mutex_new(vlc_object_t *, vlc_mutex_t *)

Never mind. The structure would likely be stored in the p_sys of the 
vlc_object and released before the object is deleted.

> The structure storage wouldn't be created (as it is now) and the mutex 
> is accessed the same way as before.
> 
>> On Tue, Jan 28, 2020, at 21:30, Rémi Denis-Courmont wrote:
>>> This adds a function to allocate a mutex that automatically gets
>>> destroyed when a module fails or terminates.
>>>
>>> I have very mixed feelings about this, as it introduces an error return
>>> where there is normally none, i.e., from vlc_mutex_init(). It also
>>> requires tedious rework to replace `vlc_mutex_t' with `vlc_mutex_t *'.
>>>
>>> For reference, kernel code entirely solves this conundrum by not
>>> requiring explicit mutex destruction (a kernel mutex does not hold any
>>> resource besides its own `struct mutex' storage space). This could also
>>> be done, but with significant reworking of the VLC threading layer and
>>> prevents reliable detection of attempt to delete a still-in-use mutex.
>>> ---
>>>   include/vlc_objects.h |  2 ++
>>>   src/misc/objres.c     | 15 +++++++++++++++
>>>   2 files changed, 17 insertions(+)
>>>
>>> diff --git a/include/vlc_objects.h b/include/vlc_objects.h
>>> index 19ee5207fc..32f23abf41 100644
>>> --- a/include/vlc_objects.h
>>> +++ b/include/vlc_objects.h
>>> @@ -295,5 +295,7 @@ VLC_API VLC_MALLOC char
>>> *vlc_obj_strdup(vlc_object_t *obj, const char *str);
>>>    */
>>>   VLC_API void vlc_obj_free(vlc_object_t *obj, void *ptr);
>>> +VLC_API VLC_MALLOC vlc_mutex_t *vlc_obj_mutex_new(vlc_object_t *obj);
>>> +
>>>   /** @} */
>>>   /** @} */
>>> diff --git a/src/misc/objres.c b/src/misc/objres.c
>>> index cac8624e7e..383dd760a8 100644
>>> --- a/src/misc/objres.c
>>> +++ b/src/misc/objres.c
>>> @@ -174,3 +174,18 @@ void vlc_obj_free(vlc_object_t *obj, void *ptr)
>>>   {
>>>       vlc_objres_remove(obj, ptr, ptrcmp);
>>>   }
>>> +
>>> +static void mutex_release(void *data)
>>> +{
>>> +    vlc_mutex_destroy(data);
>>> +}
>>> +
>>> +vlc_mutex_t *vlc_obj_mutex_new(vlc_object_t *obj)
>>> +{
>>> +    vlc_mutex_t *mtx = vlc_objres_new(sizeof (*mtx), mutex_release);
>>> +    if (mtx != NULL) {
>>> +        vlc_mutex_init(mtx);
>>> +        vlc_objres_push(obj, mtx);
>>> +    }
>>> +    return mtx;
>>> +}
>>> -- 
>>> 2.25.0
>>>
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