Andreas Rheinhardt:
> Up until now setting the input and output devices lists is guarded
> by a mutex. This prevents data races emanating from multiple concurrent
> calls to avpriv_register_devices() (triggered by multiple concurrent
> calls to avdevice_register_all()). Yet reading the lists pointers was
> done without any lock and with nonatomic variables. This means that
> there are data races in case of concurrent calls to
> av_(de)muxer_iterate() and avdevice_register_all() (but only if the
> iteration in av_(de)muxer_iterate exhausts the non-device (de)muxers).
>
> This commit fixes this by putting said pointers into atomic objects.
> Due to the unavailability of _Atomic the object is an atomic_uintptr,
> leading to ugly casts. Switching to atomics also allowed to remove
> the mutex currently used in avpriv_register_devices().
>
> Signed-off-by: Andreas Rheinhardt <andreas.rheinha...@outlook.com>
> ---
> libavformat/allformats.c | 22 +++++++++++-----------
> 1 file changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/libavformat/allformats.c b/libavformat/allformats.c
> index 5471f7c16f..cadaf057fc 100644
> --- a/libavformat/allformats.c
> +++ b/libavformat/allformats.c
> @@ -19,7 +19,7 @@
> * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301
> USA
> */
>
> -#include "libavutil/thread.h"
> +#include <stdatomic.h>
> #include "libavformat/internal.h"
> #include "avformat.h"
>
> @@ -534,18 +534,20 @@ extern const AVInputFormat ff_vapoursynth_demuxer;
> #include "libavformat/muxer_list.c"
> #include "libavformat/demuxer_list.c"
>
> -static const AVInputFormat * const *indev_list = NULL;
> -static const AVOutputFormat * const *outdev_list = NULL;
> +static atomic_uintptr_t indev_list_intptr = ATOMIC_VAR_INIT(0);
> +static atomic_uintptr_t outdev_list_intptr = ATOMIC_VAR_INIT(0);
>
> const AVOutputFormat *av_muxer_iterate(void **opaque)
> {
> static const uintptr_t size = sizeof(muxer_list)/sizeof(muxer_list[0]) -
> 1;
> uintptr_t i = (uintptr_t)*opaque;
> const AVOutputFormat *f = NULL;
> + uintptr_t tmp;
>
> if (i < size) {
> f = muxer_list[i];
> - } else if (outdev_list) {
> + } else if (tmp = atomic_load_explicit(&outdev_list_intptr,
> memory_order_relaxed)) {
> + const AVOutputFormat *const *outdev_list = (const AVOutputFormat
> *const *)tmp;
> f = outdev_list[i - size];
> }
>
> @@ -559,10 +561,12 @@ const AVInputFormat *av_demuxer_iterate(void **opaque)
> static const uintptr_t size =
> sizeof(demuxer_list)/sizeof(demuxer_list[0]) - 1;
> uintptr_t i = (uintptr_t)*opaque;
> const AVInputFormat *f = NULL;
> + uintptr_t tmp;
>
> if (i < size) {
> f = demuxer_list[i];
> - } else if (indev_list) {
> + } else if (tmp = atomic_load_explicit(&indev_list_intptr,
> memory_order_relaxed)) {
> + const AVInputFormat *const *indev_list = (const AVInputFormat *const
> *)tmp;
> f = indev_list[i - size];
> }
>
> @@ -571,12 +575,8 @@ const AVInputFormat *av_demuxer_iterate(void **opaque)
> return f;
> }
>
> -static AVMutex avpriv_register_devices_mutex = AV_MUTEX_INITIALIZER;
> -
> void avpriv_register_devices(const AVOutputFormat * const o[], const
> AVInputFormat * const i[])
> {
> - ff_mutex_lock(&avpriv_register_devices_mutex);
> - outdev_list = o;
> - indev_list = i;
> - ff_mutex_unlock(&avpriv_register_devices_mutex);
> + atomic_store_explicit(&outdev_list_intptr, (uintptr_t)o,
> memory_order_relaxed);
> + atomic_store_explicit(&indev_list_intptr, (uintptr_t)i,
> memory_order_relaxed);
> }
>
Will apply this patch tomorrow unless there are objections.
- Andreas
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