gstreamer: Use dedicated lock for request queues

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@ideasonboard.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>

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());
+	std::unique_ptr<RequestWrap> wrap;
-	queuedRequests_.pop();
+
+	{
+		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);