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1e27ea63af
The `gst_v4l2_buffer_pool_dqbuf` function contains this ominous comment:
/* get our GstBuffer with that index from the pool, if the buffer was
* outstanding we have a serious problem.
*/
outbuf = pool->buffers[group->buffer.index];
Unfortunately it is common for buffers in _output_ buffer pools to be
both queued and outstanding at the same time. This can happen if the
upstream element keeps a reference to the buffer, or in an encoder
element itself when it keeps a reference to the input buffer for each
frame.
Since the current code doesn't handle this case properly we can end up
with crashes in other elements such as:
(gst-launch-1.0:32559): CRITICAL **: 17:33:35.740: gst_video_frame_map_id: assertion 'GST_IS_BUFFER (buffer)' failed
and:
(gst-launch-1.0:231): GStreamer-CRITICAL **: 00:16:20.882: write map requested on non-writable buffer
Both these crashes are caused by a race condition related to releasing
the same buffer twice from two different threads. If a buffer is queued
and outstanding this situation is possible:
**Thread 1**
- Calls `gst_buffer_unref` decrementing the reference count to zero.
- The core GstBufferPool object marks the buffer non-outstanding.
- Calls the V4L2 release buffer function.
- If the buffer is _not_ queued:
- Release it back to the free pool (containing non-queued buffers).
**Thread 2**
- Dequeues the queued output buffer.
- Marks the buffer as not queued.
- If the buffer is _not_ outstanding:
- Calls the V4L2 release buffer function.
- Release it back to the free pool (containing non-queued buffers).
If both of these threads run at exactly the same time there is a small
window where the buffer is marked both not outstanding and not queued
but before it has been released. In this case the buffer will be freed
twice causing the above crashes.
Unfortunately the variable recording whether a buffer is outstanding is
part of the core `GstBuffer` object and is managed by `GstBufferPool` so
it's not as straightforward as adding a mutex. Instead we can fix this
by additionally recording the buffer state in `GstV4l2BufferPool`, and
handle "internal" and "external" buffer release separately so we can
detect when a buffer becomes not outstanding.
In the new solution:
- The "external" buffer pool release and the "dqbuf" functions
atomically update the buffer state and determine if a buffer is still
queued or outstanding.
- Subsequent code and a new
`gst_v4l2_buffer_pool_complete_release_buffer` function can proceed to
release (or not) a buffer knowing that it's not racing with another
thread.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1010>
|
||
|---|---|---|
| .. | ||
| ext | ||
| gstv4l2.c | ||
| gstv4l2allocator.c | ||
| gstv4l2allocator.h | ||
| gstv4l2bufferpool.c | ||
| gstv4l2bufferpool.h | ||
| gstv4l2codec.c | ||
| gstv4l2codec.h | ||
| gstv4l2colorbalance.c | ||
| gstv4l2colorbalance.h | ||
| gstv4l2deviceprovider.c | ||
| gstv4l2deviceprovider.h | ||
| gstv4l2element.c | ||
| gstv4l2elements.h | ||
| gstv4l2fwhtenc.c | ||
| gstv4l2fwhtenc.h | ||
| gstv4l2h263enc.c | ||
| gstv4l2h263enc.h | ||
| gstv4l2h264codec.c | ||
| gstv4l2h264codec.h | ||
| gstv4l2h264enc.c | ||
| gstv4l2h264enc.h | ||
| gstv4l2h265codec.c | ||
| gstv4l2h265codec.h | ||
| gstv4l2h265enc.c | ||
| gstv4l2h265enc.h | ||
| gstv4l2jpegenc.c | ||
| gstv4l2jpegenc.h | ||
| gstv4l2mpeg2codec.c | ||
| gstv4l2mpeg2codec.h | ||
| gstv4l2mpeg4codec.c | ||
| gstv4l2mpeg4codec.h | ||
| gstv4l2mpeg4enc.c | ||
| gstv4l2mpeg4enc.h | ||
| gstv4l2object.c | ||
| gstv4l2object.h | ||
| gstv4l2radio.c | ||
| gstv4l2radio.h | ||
| gstv4l2sink.c | ||
| gstv4l2sink.h | ||
| gstv4l2src.c | ||
| gstv4l2src.h | ||
| gstv4l2transform.c | ||
| gstv4l2transform.h | ||
| gstv4l2tuner.c | ||
| gstv4l2tuner.h | ||
| gstv4l2videodec.c | ||
| gstv4l2videodec.h | ||
| gstv4l2videoenc.c | ||
| gstv4l2videoenc.h | ||
| gstv4l2vidorient.c | ||
| gstv4l2vidorient.h | ||
| gstv4l2vp8codec.c | ||
| gstv4l2vp8codec.h | ||
| gstv4l2vp8enc.c | ||
| gstv4l2vp8enc.h | ||
| gstv4l2vp9codec.c | ||
| gstv4l2vp9codec.h | ||
| gstv4l2vp9enc.c | ||
| gstv4l2vp9enc.h | ||
| meson.build | ||
| README | ||
| tuner.c | ||
| tuner.h | ||
| tunerchannel.c | ||
| tunerchannel.h | ||
| tunernorm.c | ||
| tunernorm.h | ||
| v4l2-utils.c | ||
| v4l2-utils.h | ||
| v4l2_calls.c | ||
v4l2 plugins
============
The idea is a bit the same as the idea for the v4l1 plugins. We want
one generic v4l2element, and a few child objects (probably only two:
v4l2src and v4l2sink):
/-------- v4l2src
v4l2element ---=
\-------- v4l2sink
Both v4l2src and v4l2sink have a uncompressed and a compressed
recording-/playback-mode. Since this is all part of v4l2, the 'client'
of these elements, i.e. an application using v4l2src/v4l2sink, will
hardly notice this. All capsnego stuff is done inside, and the plugin
knows which formats are compressed and which are not.
Please note that the v4l1 and the v4l2 plugins are *not* compatible
concerning properties. Naming has been kept the same where possible,
but in some cases, properties had to be removed or added to make
full use of v4l2.
V4L2 API: http://linux.bytesex.org/v4l2/.
http://v4l2spec.bytesex.org/
/usr/include/linux/videodev2.h or
Kernel patches available from
http://dl.bytesex.org/patches/.
Articles:
http://lwn.net/Articles/203924/