dma-buf: Introduce dma_buf_put_sync()

dma_buf_put() is supposed to invoke the release dma-buf callback for
a dma-buf and free the memory when the dma-buf's refcount reaches 0.
However, when dma_buf_put() is invoked from a kernel thread (e.g.
kthread or kworker), the dma-buf is actually freed later on in time,
asynchronously, by a worker thread.

This behavior can cause issues where memory is allocated from an ION
heap, such as a CMA heap, and released from the context of a worker
thread, and the memory is allocated soon after the call to
dma_buf_put(). In that case, it is possible for the memory to not have
been freed yet.

Thus, introduce dma_buf_put_sync(), which ensures that the buffer is
freed when the function is called, and the buffer does not have any
outstanding references.

Change-Id: Iba63c968b16669013684861afd60c0212062412e
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>

drivers/dma-buf/dma-buf.c

@@ -688,6 +688,37 @@ void dma_buf_put(struct dma_buf *dmabuf)
 }
 EXPORT_SYMBOL_GPL(dma_buf_put);
 
+/**
+ * dma_buf_put_sync - decreases refcount of the buffer
+ * @dmabuf:	[in]	buffer to reduce refcount of
+ *
+ * Uses file's refcounting done implicitly by __fput_sync().
+ *
+ * If, as a result of this call, the refcount becomes 0, the 'release' file
+ * operation related to this fd is called. It calls &dma_buf_ops.release vfunc
+ * in turn, and frees the memory allocated for dmabuf when exported.
+ *
+ * This function is different than dma_buf_put() in the sense that it guarantees
+ * that the 'release' file operation related to this fd is called, and that the
+ * memory is released, when the refcount becomes 0. dma_buf_put() does not
+ * have the same guarantee when invoked by a kernel thread (e.g. a worker
+ * thread), and the refcount reaches 0; in that case, the buffer is added to
+ * the delayed_fput_list, and freed asynchronously.
+ *
+ * This function should not be called in atomic context, and should only be
+ * called by kernel threads. If in doubt, use dma_buf_put().
+ */
+void dma_buf_put_sync(struct dma_buf *dmabuf)
+{
+	if (WARN_ON(!dmabuf || !dmabuf->file))
+		return;
+
+	might_sleep();
+
+	dma_buf_ref_mod(to_msm_dma_buf(dmabuf), -1);
+	__fput_sync(dmabuf->file);
+}
+
 /**
  * dma_buf_attach - Add the device to dma_buf's attachments list; optionally,
  * calls attach() of dma_buf_ops to allow device-specific attach functionality

include/linux/dma-buf.h

@@ -531,6 +531,7 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info);
 int dma_buf_fd(struct dma_buf *dmabuf, int flags);
 struct dma_buf *dma_buf_get(int fd);
 void dma_buf_put(struct dma_buf *dmabuf);
+void dma_buf_put_sync(struct dma_buf *dmabuf);
 
 struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *,
 					enum dma_data_direction);