[PATCH] i_sem-less generic_file_write for O_DIRECT & XFS

There are a number of places where generic_file_write could be used if
it didn't take i_sem by itself or where it doesn't need it:

(1)	XFS needs to do some preparation in it's write file operation
	(most notably taking it's own per-inode locks), after taking
	i_sem, but before calling the guts of generic_file_write.
(2)	blockdevices are happy with parralel get_block() operations,
	and there is nothing i_sem could protect.

This patch introduces a generic_file_write_nolock and makes the good old
generic_file_write a wrapper around.  The block device code is switched
over to it.

fs/block_dev.c

@@ -874,7 +874,7 @@ struct file_operations def_blk_fops = {
 	release:	blkdev_close,
 	llseek:		block_llseek,
 	read:		generic_file_read,
-	write:		generic_file_write,
+	write:		generic_file_write_nolock,
 	mmap:		generic_file_mmap,
 	fsync:		block_fsync,
 	ioctl:		blkdev_ioctl,

include/linux/fs.h

@@ -1241,6 +1241,7 @@ extern int generic_file_mmap(struct file *, struct vm_area_struct *);
 extern int file_read_actor(read_descriptor_t * desc, struct page *page, unsigned long offset, unsigned long size);
 extern ssize_t generic_file_read(struct file *, char *, size_t, loff_t *);
 extern ssize_t generic_file_write(struct file *, const char *, size_t, loff_t *);
+extern ssize_t generic_file_write_nolock(struct file *, const char *, size_t, loff_t *);
 extern ssize_t generic_file_sendfile(struct file *, struct file *, loff_t *, size_t);
 extern void do_generic_file_read(struct file *, loff_t *, read_descriptor_t *, read_actor_t);
 ssize_t generic_file_direct_IO(int rw, struct inode *inode, char *buf,

kernel/ksyms.c

@@ -222,6 +222,7 @@ EXPORT_SYMBOL(generic_file_read);
 EXPORT_SYMBOL(generic_file_sendfile);
 EXPORT_SYMBOL(do_generic_file_read);
 EXPORT_SYMBOL(generic_file_write);
+EXPORT_SYMBOL(generic_file_write_nolock);
 EXPORT_SYMBOL(generic_file_mmap);
 EXPORT_SYMBOL(generic_ro_fops);
 EXPORT_SYMBOL(file_lock_list);

mm/filemap.c

@@ -1812,9 +1812,8 @@ inline void remove_suid(struct dentry *dentry)
  * it for writing by marking it dirty.
  *							okir@monad.swb.de
  */
-ssize_t
-generic_file_write(struct file *file, const char *buf,
-		size_t count, loff_t *ppos)
+ssize_t generic_file_write_nolock(struct file *file, const char *buf,
+				  size_t count, loff_t *ppos)
 {
 	struct address_space * mapping = file->f_dentry->d_inode->i_mapping;
 	struct address_space_operations *a_ops = mapping->a_ops;
@@ -1829,18 +1828,15 @@ generic_file_write(struct file *file, const char *buf,
 	unsigned	bytes;
 	time_t		time_now;
 
-	if (unlikely((ssize_t) count < 0))
+	if (unlikely((ssize_t)count < 0))
 		return -EINVAL;
 
 	if (unlikely(!access_ok(VERIFY_READ, buf, count)))
 		return -EFAULT;
 
-	down(&inode->i_sem);
 	pos = *ppos;
-	if (unlikely(pos < 0)) {
-		err = -EINVAL;
-		goto out;
-	}
+	if (unlikely(pos < 0))
+		return -EINVAL;
 
 	if (unlikely(file->f_error)) {
 		err = file->f_error;
@@ -2037,6 +2033,18 @@ generic_file_write(struct file *file, const char *buf,
 out_status:	
 	err = written ? written : status;
 out:
+	return err;
+}
+
+ssize_t generic_file_write(struct file *file, const char *buf,
+			   size_t count, loff_t *ppos)
+{
+	struct inode	*inode = file->f_dentry->d_inode->i_mapping->host;
+	int		err;
+
+	down(&inode->i_sem);
+	err = generic_file_write_nolock(file, buf, count, ppos);
 	up(&inode->i_sem);
+
 	return err;
 }