[libcamera-devel] [PATCH v2 08/12] gstreamer: Use dedicated lock for request queues

[Laurent Pinchart]

Add a new lock to the GstLibcameraSrcState class to protect the queued
and completed requests queues. This replaces the GstObject lock, and
minimizes the lock contention between the request completion handler and
the task run handler as the former must run as fast as possible.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne at collabora.com>
---
Changes since v1:

- Add comment about lock_
---
 src/gstreamer/gstlibcamerasrc.cpp | 44 +++++++++++++++++++++++--------
 1 file changed, 33 insertions(+), 11 deletions(-)

diff --git a/src/gstreamer/gstlibcamerasrc.cpp b/src/gstreamer/gstlibcamerasrc.cpp
index 8c1cd7017d98..6f9a03c515d2 100644
--- a/src/gstreamer/gstlibcamerasrc.cpp
+++ b/src/gstreamer/gstlibcamerasrc.cpp
@@ -32,6 +32,8 @@
 #include <queue>
 #include <vector>
 
+#include <libcamera/base/mutex.h>
+
 #include <libcamera/camera.h>
 #include <libcamera/camera_manager.h>
 #include <libcamera/control_ids.h>
@@ -111,8 +113,18 @@ struct GstLibcameraSrcState {
 	std::shared_ptr<Camera> cam_;
 	std::unique_ptr<CameraConfiguration> config_;
 	std::vector<GstPad *> srcpads_;
-	std::queue<std::unique_ptr<RequestWrap>> queuedRequests_;
-	std::queue<std::unique_ptr<RequestWrap>> completedRequests_;
+
+	/*
+	 * Contention on this lock_ must be minimized, as it has to be taken in
+	 * the realtime-sensitive requestCompleted() handler to protect
+	 * queuedRequests_ and completedRequests_.
+	 */
+	Mutex lock_;
+	std::queue<std::unique_ptr<RequestWrap>> queuedRequests_
+		LIBCAMERA_TSA_GUARDED_BY(lock_);
+	std::queue<std::unique_ptr<RequestWrap>> completedRequests_
+		LIBCAMERA_TSA_GUARDED_BY(lock_);
+
 	guint group_id_;
 
 	void requestCompleted(Request *request);
@@ -155,12 +167,15 @@ GstStaticPadTemplate request_src_template = {
 void
 GstLibcameraSrcState::requestCompleted(Request *request)
 {
-	GLibLocker lock(GST_OBJECT(src_));
-
 	GST_DEBUG_OBJECT(src_, "buffers are ready");
 
-	std::unique_ptr<RequestWrap> wrap = std::move(queuedRequests_.front());
-	queuedRequests_.pop();
+	std::unique_ptr<RequestWrap> wrap;
+
+	{
+		MutexLocker locker(lock_);
+		wrap = std::move(queuedRequests_.front());
+		queuedRequests_.pop();
+	}
 
 	g_return_if_fail(wrap->request_.get() == request);
 
@@ -183,7 +198,10 @@ GstLibcameraSrcState::requestCompleted(Request *request)
 		wrap->latency_ = sys_now - timestamp;
 	}
 
-	completedRequests_.push(std::move(wrap));
+	{
+		MutexLocker locker(lock_);
+		completedRequests_.push(std::move(wrap));
+	}
 
 	gst_task_resume(src_->task);
 }
@@ -289,16 +307,17 @@ gst_libcamera_src_task_run(gpointer user_data)
 	}
 
 	if (wrap) {
-		GLibLocker lock(GST_OBJECT(self));
 		GST_TRACE_OBJECT(self, "Requesting buffers");
 		state->cam_->queueRequest(wrap->request_.get());
+
+		MutexLocker locker(state->lock_);
 		state->queuedRequests_.push(std::move(wrap));
 
 		/* The RequestWrap will be deleted in the completion handler. */
 	}
 
 	{
-		GLibLocker lock(GST_OBJECT(self));
+		MutexLocker locker(state->lock_);
 
 		if (!state->completedRequests_.empty()) {
 			wrap = std::move(state->completedRequests_.front());
@@ -358,7 +377,7 @@ gst_libcamera_src_task_run(gpointer user_data)
 		bool do_pause;
 
 		{
-			GLibLocker lock(GST_OBJECT(self));
+			MutexLocker locker(state->lock_);
 			do_pause = state->completedRequests_.empty();
 		}
 
@@ -513,7 +532,10 @@ gst_libcamera_src_task_leave([[maybe_unused]] GstTask *task,
 
 	state->cam_->stop();
 
-	state->completedRequests_ = {};
+	{
+		MutexLocker locker(state->lock_);
+		state->completedRequests_ = {};
+	}
 
 	for (GstPad *srcpad : state->srcpads_)
 		gst_libcamera_pad_set_pool(srcpad, nullptr);
-- 
Regards,

Laurent Pinchart