dma-buf: add ref counting for module as exporter

Add reference counting on a kernel module that exports dma-buf and
implements its operations. This prevents the module from being unloaded
while DMABUF file is in use.

The original patch [1] was submitted by Tomasz Stanislawski, but this
is a simpler way to do it.

v3: call module_put() as late as possible, per gregkh's comment.
v2: move owner to struct dma_buf, and use DEFINE_DMA_BUF_EXPORT_INFO
    macro to simplify the change.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>

[1]: https://lkml.org/lkml/2012/8/8/163

1 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index 2f0b431b73e0..f98bd7068d55 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -115,6 +115,8 @@ struct dma_buf_ops {
  * @attachments: list of dma_buf_attachment that denotes all devices attached.
  * @ops: dma_buf_ops associated with this buffer object.
  * @exp_name: name of the exporter; useful for debugging.
+ * @owner: pointer to exporter module; used for refcounting when exporter is a
+ *         kernel module.
  * @list_node: node for dma_buf accounting and debugging.
  * @priv: exporter specific private data for this buffer object.
  * @resv: reservation object linked to this dma-buf
@@ -129,6 +131,7 @@ struct dma_buf {
         unsigned vmapping_counter;
         void *vmap_ptr;
         const char *exp_name;
+        struct module *owner;
         struct list_head list_node;
         void *priv;
         struct reservation_object *resv;
@@ -164,7 +167,8 @@ struct dma_buf_attachment {
 
 /**
  * struct dma_buf_export_info - holds information needed to export a dma_buf
- * @exp_name:   name of the exporting module - useful for debugging.
+ * @exp_name:   name of the exporter - useful for debugging.
+ * @owner:      pointer to exporter module - used for refcounting kernel module
  * @ops:        Attach allocator-defined dma buf ops to the new buffer
  * @size:       Size of the buffer
  * @flags:      mode flags for the file
@@ -176,6 +180,7 @@ struct dma_buf_attachment {
  */
 struct dma_buf_export_info {
         const char *exp_name;
+        struct module *owner;
         const struct dma_buf_ops *ops;
         size_t size;
         int flags;
@@ -187,7 +192,8 @@ struct dma_buf_export_info {
  * helper macro for exporters; zeros and fills in most common values
  */
 #define DEFINE_DMA_BUF_EXPORT_INFO(a)   \
-        struct dma_buf_export_info a = { .exp_name = KBUILD_MODNAME }
+        struct dma_buf_export_info a = { .exp_name = KBUILD_MODNAME, \
+                                         .owner = THIS_MODULE }
 
 /**
  * get_dma_buf - convenience wrapper for get_file.