[FFmpeg-devel] [PATCH/RFC] Trac ticket 2363

[Michael Niedermayer]

On Sun, Mar 17, 2013 at 06:27:28PM -0300, James Almer wrote:
> lavu/atomic.c is not compiling with gcc when using -march=i386 and w32threads 
> or os2threads because __sync_val_compare_and_swap() is not defined and the 
> fallback implementation currently only supports pthreads.
> Mingw-w64 has MemoryBarrier() defined even when using -march=i386, so this is 
> not as much of a problem as it is with mingw32.
> 
> I was able to make it work using the pthreads wrappers from libavcodec, but 
> I'm not sure if this is a proper fix or not since threading is not my field of 
> expertise.
> 
> I'm attaching two versions of the proposed patch. Both are essentially the 
> same, but one alters the pthreads implementation for the sake of cleaner code.
> I didn't test with the pthreads wrapper for os2threads because i have no way 
> to do it, but in theory it should work as well.
> 
> I'll send an actual patch once (and if) one of these is accepted.
> 
> Regards.

>  atomic.c |   27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)
> d2f851be5ec4d25c4b655b6a5cef60594edf989e  atomic.diff
> diff --git a/libavutil/atomic.c b/libavutil/atomic.c
> index 7a701e1..12537ad 100644
> --- a/libavutil/atomic.c
> +++ b/libavutil/atomic.c
> @@ -22,38 +22,50 @@
>  
>  #if !HAVE_MEMORYBARRIER && !HAVE_SYNC_VAL_COMPARE_AND_SWAP && !HAVE_MACHINE_RW_BARRIER
>  
> -#if HAVE_PTHREADS
> +#if HAVE_THREADS
>  
> +#if HAVE_PTHREADS
>  #include <pthread.h>
> +#elif HAVE_W32THREADS
> +#include "w32pthreads.h"
> +#elif HAVE_OS2THREADS
> +#include "os2threads.h"
> +#endif
>  
> -static pthread_mutex_t atomic_lock = PTHREAD_MUTEX_INITIALIZER;
> +static pthread_mutex_t atomic_lock;
>  
>  int avpriv_atomic_int_get(volatile int *ptr)
>  {
>      int res;
>  
> +    pthread_mutex_init(&atomic_lock, NULL);
>      pthread_mutex_lock(&atomic_lock);
>      res = *ptr;
>      pthread_mutex_unlock(&atomic_lock);
> +    pthread_mutex_destroy(&atomic_lock);

one possible solution would look something like:

+static int volatile init;

 int avpriv_atomic_int_get(volatile int *ptr)
 {
     int res;

+    if(!init) {
+        pthread_mutex_init(&atomic_lock, NULL);
+        init = 1;
+    }

     pthread_mutex_lock(&atomic_lock);
     res = *ptr;
     pthread_mutex_unlock(&atomic_lock);
 }

this has 2 issues
theres a unlikely race on allocation and we dont bother
destroying the mutex ever

also, a comment from some win32 expert if there is a better way would
be welcome ...

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When you are offended at any man's fault, turn to yourself and study your
own failings. Then you will forget your anger. -- Epictetus
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