4 files changed, 52 insertions, 33 deletions

diff --git a/include/libcamera/internal/camera.h b/include/libcamera/internal/camera.h
index 9f5adac1..1ebfde65 100644
--- a/include/libcamera/internal/camera.h
+++ b/include/libcamera/internal/camera.h
@@ -50,6 +50,7 @@ private:
 		CameraRunning,
 	};
 
+	bool isAcquired() const;
 	bool isRunning() const;
 	int isAccessAllowed(State state, bool allowDisconnected = false,
 			    const char *from = __builtin_FUNCTION()) const;
diff --git a/include/libcamera/internal/pipeline_handler.h b/include/libcamera/internal/pipeline_handler.h
index b473eb70..17e5c892 100644
--- a/include/libcamera/internal/pipeline_handler.h
+++ b/include/libcamera/internal/pipeline_handler.h
@@ -44,8 +44,8 @@ public:
 	MediaDevice *acquireMediaDevice(DeviceEnumerator *enumerator,
 					const DeviceMatch &dm);
 
-	bool lock();
-	void unlock();
+	bool acquire();
+	void release();
 
 	virtual CameraConfiguration *generateConfiguration(Camera *camera,
 		const StreamRoles &roles) = 0;
@@ -74,7 +74,7 @@ protected:
 	CameraManager *manager_;
 
 private:
-	void unlockLocked();
+	void unlockMediaDevices();
 
 	void mediaDeviceDisconnected(MediaDevice *media);
 	virtual void disconnect();
@@ -85,7 +85,7 @@ private:
 	const char *name_;
 
 	Mutex lock_;
-	bool lockOwner_;
+	unsigned int useCount_;
 
 	friend class PipelineHandlerFactory;
 };
diff --git a/src/libcamera/camera.cpp b/src/libcamera/camera.cpp
index 400a7cf0..c10972cf 100644
--- a/src/libcamera/camera.cpp
+++ b/src/libcamera/camera.cpp
@@ -424,6 +424,11 @@ static const char *const camera_state_names[] = {
 	"Running",
 };
 
+bool Camera::Private::isAcquired() const
+{
+	return state_.load(std::memory_order_acquire) == CameraRunning;
+}
+
 bool Camera::Private::isRunning() const
 {
 	return state_.load(std::memory_order_acquire) == CameraRunning;
@@ -727,7 +732,7 @@ int Camera::exportFrameBuffers(Stream *stream,
  * not blocking, if the device has already been acquired (by the same or another
  * process) the -EBUSY error code is returned.
  *
- * Acquiring a camera will limit usage of any other camera(s) provided by the
+ * Acquiring a camera may limit usage of any other camera(s) provided by the
  * same pipeline handler to the same instance of libcamera. The limit is in
  * effect until all cameras from the pipeline handler are released. Other
  * instances of libcamera can still list and examine the cameras but will fail
@@ -755,7 +760,7 @@ int Camera::acquire()
 	if (ret < 0)
 		return ret == -EACCES ? -EBUSY : ret;
 
-	if (!d->pipe_->lock()) {
+	if (!d->pipe_->acquire()) {
 		LOG(Camera, Info)
 			<< "Pipeline handler in use by another process";
 		return -EBUSY;
@@ -789,7 +794,8 @@ int Camera::release()
 	if (ret < 0)
 		return ret == -EACCES ? -EBUSY : ret;
 
-	d->pipe_->unlock();
+	if (d->isAcquired())
+		d->pipe_->release();
 
 	d->setState(Private::CameraAvailable);
 
diff --git a/src/libcamera/pipeline_handler.cpp b/src/libcamera/pipeline_handler.cpp
index 2d2bb940..844eeebc 100644
--- a/src/libcamera/pipeline_handler.cpp
+++ b/src/libcamera/pipeline_handler.cpp
@@ -63,7 +63,7 @@ LOG_DEFINE_CATEGORY(Pipeline)
  * respective factories.
  */
 PipelineHandler::PipelineHandler(CameraManager *manager)
-	: manager_(manager), lockOwner_(false)
+	: manager_(manager), useCount_(0)
 {
 }
 
@@ -138,63 +138,75 @@ MediaDevice *PipelineHandler::acquireMediaDevice(DeviceEnumerator *enumerator,
 }
 
 /**
- * \brief Lock all media devices acquired by the pipeline
+ * \brief Acquire exclusive access to the pipeline handler for the process
  *
- * This function shall not be called from pipeline handler implementation, as
- * the Camera class handles locking directly.
+ * This function locks all the media devices used by the pipeline to ensure
+ * that no other process can access them concurrently.
+ *
+ * Access to a pipeline handler may be acquired recursively from within the
+ * same process. Every successful acquire() call shall be matched with a
+ * release() call. This allows concurrent access to the same pipeline handler
+ * from different cameras within the same process.
+ *
+ * Pipeline handlers shall not call this function directly as the Camera class
+ * handles access internally.
  *
  * \context This function is \threadsafe.
  *
- * \return True if the devices could be locked, false otherwise
- * \sa unlock()
- * \sa MediaDevice::lock()
+ * \return True if the pipeline handler was acquired, false if another process
+ * has already acquired it
+ * \sa release()
  */
-bool PipelineHandler::lock()
+bool PipelineHandler::acquire()
 {
 	MutexLocker locker(lock_);
 
-	/* Do not allow nested locking in the same libcamera instance. */
-	if (lockOwner_)
-		return false;
+	if (useCount_) {
+		++useCount_;
+		return true;
+	}
 
 	for (std::shared_ptr<MediaDevice> &media : mediaDevices_) {
 		if (!media->lock()) {
-			unlock();
+			unlockMediaDevices();
 			return false;
 		}
 	}
 
-	lockOwner_ = true;
-
+	++useCount_;
 	return true;
 }
 
 /**
- * \brief Unlock all media devices acquired by the pipeline
+ * \brief Release exclusive access to the pipeline handler
+ *
+ * This function releases access to the pipeline handler previously acquired by
+ * a call to acquire(). Every release() call shall match a previous successful
+ * acquire() call. Calling this function on a pipeline handler that hasn't been
+ * acquired results in undefined behaviour.
  *
- * This function shall not be called from pipeline handler implementation, as
- * the Camera class handles locking directly.
+ * Pipeline handlers shall not call this function directly as the Camera class
+ * handles access internally.
  *
  * \context This function is \threadsafe.
  *
- * \sa lock()
+ * \sa acquire()
  */
-void PipelineHandler::unlock()
+void PipelineHandler::release()
 {
 	MutexLocker locker(lock_);
 
-	unlockLocked();
+	ASSERT(useCount_);
+
+	unlockMediaDevices();
+
+	--useCount_;
 }
 
-void PipelineHandler::unlockLocked()
+void PipelineHandler::unlockMediaDevices()
 {
-	if (lockOwner_)
-		return;
-
 	for (std::shared_ptr<MediaDevice> &media : mediaDevices_)
 		media->unlock();
-
-	lockOwner_ = false;
 }
 
 /**