[FFmpeg-devel] [PATCH 2/3] avfilter/avfilter: avfilter_register() that works in O(1) time

[Stefano Sabatini]

On date Tuesday 2013-12-17 01:05:21 +0100, Michael Niedermayer encoded:
> On Mon, Dec 16, 2013 at 05:16:37PM +0100, Stefano Sabatini wrote:
> > On date Sunday 2013-12-08 03:55:22 +0100, Michael Niedermayer encoded:
> > > This also flips the order of the list
> > > avfilters should be unique and thus the order should not matter
> > > 
> > > Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
> > > ---
> > >  libavfilter/avfilter.c |    8 +++-----
> > >  1 file changed, 3 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/libavfilter/avfilter.c b/libavfilter/avfilter.c
> > > index 2567ce9..c564b45 100644
> > > --- a/libavfilter/avfilter.c
> > > +++ b/libavfilter/avfilter.c
> > > @@ -476,7 +476,6 @@ AVFilter *avfilter_get_by_name(const char *name)
> > >  
> > >  int avfilter_register(AVFilter *filter)
> > >  {
> > > -    AVFilter **f = &first_filter;
> > >      int i;
> > >  
> > >      /* the filter must select generic or internal exclusively */
> > > @@ -488,10 +487,9 @@ int avfilter_register(AVFilter *filter)
> > >                      || (!input->start_frame && !input->end_frame));
> > >      }
> > >  
> > > -    filter->next = NULL;
> > > -
> > > -    while(*f || avpriv_atomic_ptr_cas((void * volatile *)f, NULL, filter))
> > > -        f = &(*f)->next;
> > 
> > > +    do {
> > > +        filter->next = first_filter;
> > > +    } while (filter->next != avpriv_atomic_ptr_cas((void * volatile *)&first_filter, filter->next, filter));
> > 
> > why do you need a loop for this?
> 

> consider 2 libs or an app and a lib try to use avfilter_register() at
> the same time
> we need to insert our filter atomically but its 2 operations so if
> something else inserted something after we did the first step but
> before the second then we have to retry

Isn't a mutex a cleaner solution?

Anyway feel free to push it. OTOH I don't like very much the idea of
having filters listed in order inverted to that of registration, as it
messes up output of ffmpeg -filters for example.
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