[PATCH 2/9] dma-heap: Add proper kref handling on dma-buf heaps

[Yong Wu]

From: John Stultz <jstultz at google.com>

Add proper refcounting on the dma_heap structure.
While existing heaps are built-in, we may eventually
have heaps loaded from modules, and we'll need to be
able to properly handle the references to the heaps

Also moves minor tracking into the heap structure so
we can properly free things.

Signed-off-by: John Stultz <jstultz at google.com>
Signed-off-by: T.J. Mercier <tjmercier at google.com>
Signed-off-by: Yong Wu <yong.wu at mediatek.com>
[Yong: Just add comment for "minor" and "refcount"]
---
 drivers/dma-buf/dma-heap.c | 38 ++++++++++++++++++++++++++++++++++----
 include/linux/dma-heap.h   |  6 ++++++
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
index 51030f6c9d6e..dcc0e38c61fa 100644
--- a/drivers/dma-buf/dma-heap.c
+++ b/drivers/dma-buf/dma-heap.c
@@ -11,6 +11,7 @@
 #include <linux/dma-buf.h>
 #include <linux/dma-heap.h>
 #include <linux/err.h>
+#include <linux/kref.h>
 #include <linux/list.h>
 #include <linux/nospec.h>
 #include <linux/syscalls.h>
@@ -30,6 +31,8 @@
  * @heap_devt:		heap device node
  * @list:		list head connecting to list of heaps
  * @heap_cdev:		heap char device
+ * @minor:		heap device node minor number
+ * @refcount:		reference counter for this heap device
  *
  * Represents a heap of memory from which buffers can be made.
  */
@@ -40,6 +43,8 @@ struct dma_heap {
 	dev_t heap_devt;
 	struct list_head list;
 	struct cdev heap_cdev;
+	int minor;
+	struct kref refcount;
 };
 
 static LIST_HEAD(heap_list);
@@ -205,7 +210,6 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
 {
 	struct dma_heap *heap, *h, *err_ret;
 	struct device *dev_ret;
-	unsigned int minor;
 	int ret;
 
 	if (!exp_info->name || !strcmp(exp_info->name, "")) {
@@ -222,12 +226,13 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
 	if (!heap)
 		return ERR_PTR(-ENOMEM);
 
+	kref_init(&heap->refcount);
 	heap->name = exp_info->name;
 	heap->ops = exp_info->ops;
 	heap->priv = exp_info->priv;
 
 	/* Find unused minor number */
-	ret = xa_alloc(&dma_heap_minors, &minor, heap,
+	ret = xa_alloc(&dma_heap_minors, &heap->minor, heap,
 		       XA_LIMIT(0, NUM_HEAP_MINORS - 1), GFP_KERNEL);
 	if (ret < 0) {
 		pr_err("dma_heap: Unable to get minor number for heap\n");
@@ -236,7 +241,7 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
 	}
 
 	/* Create device */
-	heap->heap_devt = MKDEV(MAJOR(dma_heap_devt), minor);
+	heap->heap_devt = MKDEV(MAJOR(dma_heap_devt), heap->minor);
 
 	cdev_init(&heap->heap_cdev, &dma_heap_fops);
 	ret = cdev_add(&heap->heap_cdev, heap->heap_devt, 1);
@@ -280,12 +285,37 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
 err2:
 	cdev_del(&heap->heap_cdev);
 err1:
-	xa_erase(&dma_heap_minors, minor);
+	xa_erase(&dma_heap_minors, heap->minor);
 err0:
 	kfree(heap);
 	return err_ret;
 }
 
+static void dma_heap_release(struct kref *ref)
+{
+	struct dma_heap *heap = container_of(ref, struct dma_heap, refcount);
+
+	/* Note, we already holding the heap_list_lock here */
+	list_del(&heap->list);
+
+	device_destroy(dma_heap_class, heap->heap_devt);
+	cdev_del(&heap->heap_cdev);
+	xa_erase(&dma_heap_minors, heap->minor);
+
+	kfree(heap);
+}
+
+void dma_heap_put(struct dma_heap *h)
+{
+	/*
+	 * Take the heap_list_lock now to avoid racing with code
+	 * scanning the list and then taking a kref.
+	 */
+	mutex_lock(&heap_list_lock);
+	kref_put(&h->refcount, dma_heap_release);
+	mutex_unlock(&heap_list_lock);
+}
+
 static char *dma_heap_devnode(const struct device *dev, umode_t *mode)
 {
 	return kasprintf(GFP_KERNEL, "dma_heap/%s", dev_name(dev));
diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
index c7c29b724ad6..f3c678892c5c 100644
--- a/include/linux/dma-heap.h
+++ b/include/linux/dma-heap.h
@@ -64,4 +64,10 @@ const char *dma_heap_get_name(struct dma_heap *heap);
  */
 struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
 
+/**
+ * dma_heap_put - drops a reference to a dmabuf heap, potentially freeing it
+ * @heap: the heap whose reference count to decrement
+ */
+void dma_heap_put(struct dma_heap *heap);
+
 #endif /* _DMA_HEAPS_H */
-- 
2.25.1