[libcamera-devel] [PATCH] gstreamer: Cleanup inner scope in gst_libcamera_src_task_run()

[Nicolas Dufresne]

Le dimanche 27 mars 2022 à 17:48 +0300, Laurent Pinchart via libcamera-devel a
écrit :
> On Sun, Mar 27, 2022 at 05:36:29PM +0300, Laurent Pinchart wrote:
> > On Sun, Mar 27, 2022 at 10:25:27AM -0400, Nicolas Dufresne wrote:
> > > Le 27 mars 2022 09 h 52, Laurent Pinchart a écrit :
> > > 
> > > > The code block that checks for flow errors in
> > > > gst_libcamera_src_task_run() is located in an inner scope whose purpose
> > > > it to handling locking. The flow error handling however occurs before
> > > > the lock is taken, so there's no need to place it in the inner scope.
> > > > Move it one level up.
> > > 
> > > > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > > > ---
> > > > src/gstreamer/gstlibcamerasrc.cpp | 26 +++++++++++++-------------
> > > > 1 file changed, 13 insertions(+), 13 deletions(-)
> > > 
> > > > diff --git a/src/gstreamer/gstlibcamerasrc.cpp b/src/gstreamer/
> > > > gstlibcamerasrc.cpp
> > > > index 46fd02d207be..8016a98d6a4d 100644
> > > > --- a/src/gstreamer/gstlibcamerasrc.cpp
> > > > +++ b/src/gstreamer/gstlibcamerasrc.cpp
> > > > @@ -311,21 +311,21 @@ gst_libcamera_src_task_run(gpointer user_data)
> > > >  							srcpad, ret);
> > > >  	}
> > > > 
> > > > - 	{
> > 
> > 	^---- [1]
> > 
> > > > - 		if (ret != GST_FLOW_OK) {
> > > > - 			if (ret == GST_FLOW_EOS) {
> > > > - 				g_autoptr(GstEvent) eos = gst_event_new_eos();
> > > > - 				guint32 seqnum = gst_util_seqnum_next();
> > > > - 				gst_event_set_seqnum(eos, seqnum);
> > > > - 				for (GstPad *srcpad : state->srcpads_)
> > > 
> > > Iterating the srcpads_ is not thread safe. Won't this change remove the locking
> > > for that ?
> > 
> > The patch doesn't change locking at all as far as I can tell, there's no
> > lock taken at the beginning of the scope ([1]). There's a lock taken
> > below ([2]), was it meant to cover this iteration too ?
> 
> Also, as far as I can tell, srcpads_ is only modified in
> gst_libcamera_src_init(), gst_libcamera_src_request_new_pad() and
> gst_libcamera_src_release_pad(). The former is called at init time only,
> can the latter two (handling the .request_new_pad() and .release_pad())
> we called while the task is running ? If so there's another possible
> race, as srcpads_ is iterated earlier in this function, with no lock
> taken.

Correct, though gst_libcamera_src_request_new_pad /
gst_libcamera_src_release_pad() is called by application through
gst_element_request_pad() / gst_element_release_request_pad(), and that from
application threads, which is random thread. Notice the lock being properly
taken in request_new_pad().

> 
> > > > - 					gst_pad_push_event(srcpad, gst_event_ref(eos));
> > > > - 			} else if (ret != GST_FLOW_FLUSHING) {
> > > > - 				GST_ELEMENT_FLOW_ERROR(self, ret);
> > > > - 			}
> > > > - 			gst_task_stop(self->task);
> > > > - 			return;
> > > > + 	if (ret != GST_FLOW_OK) {
> > > > + 		if (ret == GST_FLOW_EOS) {
> > > > + 			g_autoptr(GstEvent) eos = gst_event_new_eos();
> > > > + 			guint32 seqnum = gst_util_seqnum_next();
> > > > + 			gst_event_set_seqnum(eos, seqnum);
> > > > + 			for (GstPad *srcpad : state->srcpads_)
> > > > + 				gst_pad_push_event(srcpad, gst_event_ref(eos));
> > > > + 			} else if (ret != GST_FLOW_FLUSHING) {
> > > > + 				GST_ELEMENT_FLOW_ERROR(self, ret);
> > > >  		}
> > > > + 		gst_task_stop(self->task);
> > > > + 		return;
> > > > + 	}
> > > >  
> > > > + 	{
> > > >  		/*
> > > >  		 * Here we need to decide if we want to pause. This needs to
> > > >  		 * happen in lock step with the callback thread which may want
> > 		 */
> > 		GLibLocker lock(GST_OBJECT(self));
> > 
> > 		^---- [2]
>