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Subject[PATCH RFC] dma-buf/fs Add get_[file|dma_buf]_unless_doomed
Date
In this context, a "doomed" object is an object whose refcount has reached
zero, but that has not yet been freed.

To avoid mutual refcounting vmwgfx need to have a non-refcounted pointer to
a dma-buf in a lookup structure. The pointer is removed in the dma-buf
destructor. To allow lookup-structure private locks we need
get_dma_buf_unless_doomed(). This common refcounting scenario is described
with examples in detail in the kref documentaion.
The solution with local locks is under kref_get_unless_zero().
See also kobject_get_unless_zero() and its commit message.
Since dma-bufs are using the attached file for refcounting,
get_dma_buf_unless_doomed maps directly to a get_file_unless_doomed.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
---
include/linux/dma-buf.h | 16 ++++++++++++++++
include/linux/fs.h | 15 +++++++++++++++
2 files changed, 31 insertions(+)

diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index dfac5ed..6e58144 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -162,6 +162,22 @@ static inline void get_dma_buf(struct dma_buf *dmabuf)
get_file(dmabuf->file);
}

+/**
+ * get_dma_buf_unless_doomed - convenience wrapper for get_file_unless_doomed
+ *
+ * @dmabuf: [in] pointer to dma_buf
+ *
+ * Obtain a dma-buf reference from a lookup structure that doesn't refcount
+ * the dma-buf, but synchronizes with its release method to make sure it has
+ * not been freed yet. See for example kref_get_unless_zero documentation.
+ * Returns true if refcounting succeeds, false otherwise.
+ */
+static inline bool __must_check
+get_dma_buf_unless_doomed(struct dma_buf *dmabuf)
+{
+ return get_file_unless_doomed(dmabuf->file);
+}
+
struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
struct device *dev);
void dma_buf_detach(struct dma_buf *dmabuf,
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 3f40547..a96c333 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -824,6 +824,21 @@ static inline struct file *get_file(struct file *f)
atomic_long_inc(&f->f_count);
return f;
}
+
+/**
+ * get_file_unless_doomed - convenience wrapper for get_file_unless_doomed
+ * @file: [in] pointer to file
+ *
+ * Obtain a file reference from a lookup structure that doesn't refcount
+ * the file, but synchronizes with its release method to make sure it has
+ * not been freed yet. See for example kref_get_unless_zero documentation.
+ * Returns true if refcounting succeeds, false otherwise.
+ */
+static inline bool __must_check get_file_unless_doomed(struct file *f)
+{
+ return atomic_long_inc_not_zero(&f->f_count) != 0L;
+}
+
#define fput_atomic(x) atomic_long_add_unless(&(x)->f_count, -1, 1)
#define file_count(x) atomic_long_read(&(x)->f_count)

--
1.7.10.4

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