fs: Split fcntl_rw_hint()

Split fcntl_rw_hint() such that there is one helper function per fcntl.
Use READ_ONCE() and WRITE_ONCE() to access the i_write_hint member
instead of protecting such accesses with the inode lock. READ_ONCE() is
not used in I/O path code that reads i_write_hint. Users who want
F_SET_RW_HINT to affect I/O need to make sure that F_SET_RW_HINT has
completed before I/O is submitted that should use the configured write
hint.

Cc: Christoph Hellwig <[e-mail]>
Suggested-by: Christoph Hellwig <[e-mail]>
Cc: Kanchan Joshi <[e-mail]>
Cc: Jeff Layton <[e-mail]>
Cc: Chuck Lever <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/fcntl.c

···
 	}
 }
 
-static long fcntl_rw_hint(struct file *file, unsigned int cmd,
-			  unsigned long arg)
+static long fcntl_get_rw_hint(struct file *file, unsigned int cmd,
+			      unsigned long arg)
+{
+	struct inode *inode = file_inode(file);
+	u64 __user *argp = (u64 __user *)arg;
+	u64 hint = READ_ONCE(inode->i_write_hint);
+
+	if (copy_to_user(argp, &hint, sizeof(*argp)))
+		return -EFAULT;
+	return 0;
+}
+
+static long fcntl_set_rw_hint(struct file *file, unsigned int cmd,
+			      unsigned long arg)
 {
 	struct inode *inode = file_inode(file);
 	u64 __user *argp = (u64 __user *)arg;
 	u64 hint;
 
-	switch (cmd) {
-	case F_GET_RW_HINT:
-		hint = inode->i_write_hint;
-		if (copy_to_user(argp, &hint, sizeof(*argp)))
-			return -EFAULT;
-		return 0;
-	case F_SET_RW_HINT:
-		if (copy_from_user(&hint, argp, sizeof(hint)))
-			return -EFAULT;
-		if (!rw_hint_valid(hint))
-			return -EINVAL;
-
-		inode_lock(inode);
-		inode->i_write_hint = hint;
-		inode_unlock(inode);
-		return 0;
-	default:
+	if (copy_from_user(&hint, argp, sizeof(hint)))
+		return -EFAULT;
+	if (!rw_hint_valid(hint))
 		return -EINVAL;
-	}
+
+	WRITE_ONCE(inode->i_write_hint, hint);
+
+	return 0;
 }
 
 static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
···
 		err = memfd_fcntl(filp, cmd, argi);
 		break;
 	case F_GET_RW_HINT:
+		err = fcntl_get_rw_hint(filp, cmd, arg);
+		break;
 	case F_SET_RW_HINT:
-		err = fcntl_rw_hint(filp, cmd, arg);
+		err = fcntl_set_rw_hint(filp, cmd, arg);
 		break;
 	default:
 		break;