On 10/18/2017 03:28 PM, Mark Thompson wrote:
On 18/10/17 08:46, Jorge Ramirez wrote:
On 10/18/2017 12:34 AM, Mark Thompson wrote:
int ff_v4l2_m2m_codec_end(AVCodecContext *avctx)
{
- V4L2m2mContext* s = avctx->priv_data;
- int ret;
+ V4L2m2mContext *m2m, *s = avctx->priv_data;
+ int i, ret;
ret = ff_v4l2_context_set_status(&s->output, VIDIOC_STREAMOFF);
if (ret)
@@ -325,12 +325,31 @@ int ff_v4l2_m2m_codec_end(AVCodecContext *avctx)
av_log(avctx, AV_LOG_ERROR, "VIDIOC_STREAMOFF %s\n", s->capture.name);
ff_v4l2_context_release(&s->output);
+ sem_destroy(&s->refsync);
- if (atomic_load(&s->refcount))
- av_log(avctx, AV_LOG_ERROR, "ff_v4l2m2m_codec_end leaving pending
buffers\n");
+ if (atomic_load(&s->refcount)) {
+ av_log(avctx, AV_LOG_DEBUG, "ff_v4l2m2m_codec_end, referenced buffers %d\n",
atomic_load(&s->refcount));
+
+ /* We are about to free the private data while the user still has
references to the buffers.
+ * This means that when the user releases those buffers, the
v4l2_free_buffer callback will be pointing to free'd memory.
+ * Duplicate the m2m context and update the buffers.
+ */
+ m2m = av_mallocz(sizeof(*m2m));
+ if (m2m) {
+ memcpy(m2m, s, sizeof(V4L2m2mContext));
+ for (i = 0; i < s->capture.num_buffers; i++)
+ s->capture.buffers[i].m2m = m2m;
+
+ return 0;
+ }
No.
The user can call av_buffer_unref() and therefore v4l2_free_buffer()
asynchronously on another thread while this manoeuvre is in progress.
right. as a matter of fact, this synchronization can not be done using private
data (the opaque structure passed to the callback must contain all the required
information (and updated!) at any time). On top of this being hard to implement
would complicate the current code quite a bit making it also very hard to
maintain. So agreed.
So back to a previous proposal, would the flag below be acceptable? [1]
Seems sensible to me to give responsibility of the private data deallocation to
the codec itself independently of the close function since the buffers
allocated by the codec controlled by a separate thread will need access to that
data.
Put differently since the close function can not be made responsible for
deallocating mmap/queued memory in V4L2 (or even closing the kernel driver)
when the user keeps buffers, it should not be allowed to delete the codec
private data since in my view it forces the design to take a lot of unnecessary
actions for no real reason.
With the proposal below, the code remains cleaner and free of race conditions.
[1] ..
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 52cc5b0..0a3ca0c 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -982,6 +982,11 @@ typedef struct RcOverride{
* Do not reset ASS ReadOrder field on flush (subtitles decoding)
*/
#define AV_CODEC_FLAG2_RO_FLUSH_NOOP (1 << 30)
+/**
+ * Closing the codec doesnt release the context priv_data (it becomes the codec
+ * responsibility)
+ */
+#define AV_CODEC_FLAG2_PRESERVE_PRIVDATA_ONCLOSE (1 << 31)
/* Unsupported options :
* Syntax Arithmetic coding (SAC)
diff --git a/libavcodec/utils.c b/libavcodec/utils.c
index 9551f31..b6c5adf 100644
--- a/libavcodec/utils.c
+++ b/libavcodec/utils.c
@@ -1211,7 +1211,9 @@ av_cold int avcodec_close(AVCodecContext *avctx)
if (avctx->priv_data && avctx->codec && avctx->codec->priv_class)
av_opt_free(avctx->priv_data);
av_opt_free(avctx);
- av_freep(&avctx->priv_data);
+ if (!(avctx->flags2 & AV_CODEC_FLAG2_PRESERVE_PRIVDATA_ONCLOSE))
+ av_freep(&avctx->priv_data);
+
I'm not seeing the motivation for this at all.
right. Yes, the motivation is implicit in the way v4l2 support was
implemented (capture and output context are allocated on priv_data (they
are not pointers) to enable navigating the data structures using
container_of all over the code).
this makes the code not only fast and less prone to leaks but also
easier to read and maintain (it is a pattern used in many drivers)
The v4l2 implementation only allocates two chunks of heap to accommodate
all of its buffers. So leaks are extremely unlikely.
Anything which possibly needs to persist beyond the codec close will be
refcounted (with references from the codec instance itself and from the
outstanding output frames, as you have), so it should be stored in a refcounted
buffer.
Not if the codec puts all its information on private_data as is done in
v4l2.
That's how all other components with this problem work, so why does v4l2 want
special API to behave differently?
Given the use case we are implementing against, I think it is sensible
to give the codec the 'option' to handle its private_data (ie, the patch
above)
But I would also like to understand your concern before replacing the
structures with pointers and rewriting most of the code.
I think that the current codec API is not complete and the fact that
reference counting of buffers after codec close must be implemented (if
this was actually the reason) is probably symptom.
So I am not really proposing an especial API for v4l2 but an enhancement
of the current one to make it easier to use.
But let me explain further
The API itself is quite common:
- provides an interface to allocate private data with a size.
- provides generic interface (close) to deallocate private_data _along_
with everything else.
This is standard to many frameworks but those frameworks typically
expect the close function to release all allocated memory done during init
However the ffmpeg use case requires that closing the codec doesnt
release all memory (ie buffers); that can't be done unless
i) private_data persists under codec control (patch above so each codec
can select whether to manage the priv_date lifetime or not) OR
ii) we avoid placing critical stuff on priv_data and implement reference
counting to anything the codec needs for its buffers houskeeping.
I am repeating myself here but there are two ways to implement the use
case above:
1. an opaque on the buffer allocation callback function that carries all
the information (codec state, file descriptors, whatever) that the
buffer needs to do the house keeping.
2. an opaque on the buffer allocation callback function that points to
the codec private_data (therefore gaining access to all the codec
information that it needs).
why is it a problem to provide both and let each codec chose depending
on its implementation?
In the case of v4l2:
1. the code is navigated by using container_of (we could use pointers
instead but I think that adds noise to the definition of the structures)
2. each capture buffer released by the client needs to know
a) if the codec was closed
b) if the codec is reinitializing (dequeing all buffers to enqueue
buffers with a new size again)
c) if the buffer needs to be queued again
a) if the codec was closed it needs to know if it is the last buffer in
which case it unmaps all the buffers from the process and asks the
driver to release them.
b) it needs to know if it is the last one to allow the re-initialization
to continue (post a semaphore)
c) if we are still streaming, it needs access to the driver to queue again.
All this can be maintained easily in private_data without adding any
complexity really.
- Mark
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