dma-buf: Change locking policy for mmap()

Change locking policy of mmap() callback, making exporters responsible
for handling dma-buf reservation locking. Previous locking policy stated
that dma-buf is locked for both importers and exporters by the dma-buf
core, which caused a deadlock problem for DRM drivers in a case of
self-imported dma-bufs which required to take the lock from the DRM
exporter side.
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230529223935.2672495-6-dmitry.osipenko@collabora.com

drivers/dma-buf/dma-buf.c

@@ -131,7 +131,6 @@ static struct file_system_type dma_buf_fs_type = {
 static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
 {
 	struct dma_buf *dmabuf;
-	int ret;
 
 	if (!is_dma_buf_file(file))
 		return -EINVAL;
@@ -147,11 +146,7 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
 	    dmabuf->size >> PAGE_SHIFT)
 		return -EINVAL;
 
-	dma_resv_lock(dmabuf->resv, NULL);
-	ret = dmabuf->ops->mmap(dmabuf, vma);
-	dma_resv_unlock(dmabuf->resv);
-
-	return ret;
+	return dmabuf->ops->mmap(dmabuf, vma);
 }
 
 static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
@@ -850,6 +845,7 @@ static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
  *     - &dma_buf_ops.release()
  *     - &dma_buf_ops.begin_cpu_access()
  *     - &dma_buf_ops.end_cpu_access()
+ *     - &dma_buf_ops.mmap()
  *
  * 2. These &dma_buf_ops callbacks are invoked with locked dma-buf
  *    reservation and exporter can't take the lock:
@@ -858,7 +854,6 @@ static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
  *     - &dma_buf_ops.unpin()
  *     - &dma_buf_ops.map_dma_buf()
  *     - &dma_buf_ops.unmap_dma_buf()
- *     - &dma_buf_ops.mmap()
  *     - &dma_buf_ops.vmap()
  *     - &dma_buf_ops.vunmap()
  *
@@ -1463,8 +1458,6 @@ EXPORT_SYMBOL_NS_GPL(dma_buf_end_cpu_access, DMA_BUF);
 int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 		 unsigned long pgoff)
 {
-	int ret;
-
 	if (WARN_ON(!dmabuf || !vma))
 		return -EINVAL;
 
@@ -1485,11 +1478,7 @@ int dma_buf_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma,
 	vma_set_file(vma, dmabuf->file);
 	vma->vm_pgoff = pgoff;
 
-	dma_resv_lock(dmabuf->resv, NULL);
-	ret = dmabuf->ops->mmap(dmabuf, vma);
-	dma_resv_unlock(dmabuf->resv);
-
-	return ret;
+	return dmabuf->ops->mmap(dmabuf, vma);
 }
 EXPORT_SYMBOL_NS_GPL(dma_buf_mmap, DMA_BUF);