[PATCH] libfs: move transaction file ops into libfs

Below is an updated version of the patch which moves duplicated
transaction-based file operation code into libfs.  Since the last post, the
patch has been through a couple of iterations with Al, who suggested a
number of cleanups including locking and interface simplification.

For filesystem writers, the interface is now much simpler.  The
simple_transaction_get() helper should be part of the file op write method.
 This safely obtains the transaction request data during write(), allocates
a page for it and stores it there.  The data is returned to the caller for
potential further processing, which then makes it available for the next
read() call via simple_transaction_set().  See the selinuxfs and nfsctl
code for examples of use.

Signed-off-by: James Morris <jmorris@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index a7631bd33db1..aecce55e4f88 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1571,6 +1571,39 @@ static inline ino_t parent_ino(struct dentry *dentry)
 /* kernel/fork.c */
 extern int unshare_files(void);
 
+/* Transaction based IO helpers */
+
+/*
+ * An argresp is stored in an allocated page and holds the
+ * size of the argument or response, along with its content
+ */
+struct simple_transaction_argresp {
+	ssize_t size;
+	char data[0];
+};
+
+#define SIMPLE_TRANSACTION_LIMIT (PAGE_SIZE - sizeof(struct simple_transaction_argresp))
+
+char *simple_transaction_get(struct file *file, const char __user *buf,
+				size_t size);
+ssize_t simple_transaction_read(struct file *file, char __user *buf,
+				size_t size, loff_t *pos);
+int simple_transaction_release(struct inode *inode, struct file *file);
+
+static inline void simple_transaction_set(struct file *file, size_t n)
+{
+	struct simple_transaction_argresp *ar = file->private_data;
+
+	BUG_ON(n > SIMPLE_TRANSACTION_LIMIT);
+
+	/*
+	 * The barrier ensures that ar->size will really remain zero until
+	 * ar->data is ready for reading.
+	 */
+	smp_mb();
+	ar->size = n;
+}
+
 #ifdef CONFIG_SECURITY
 static inline char *alloc_secdata(void)
 {