fuse: add fuse_dio_lock/unlock helper functions

So far this is just a helper to remove complex locking logic out of
fuse_direct_write_iter.  Especially needed by the next patch in the series
to that adds the fuse inode cache IO mode and adds in even more locking
complexity.
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -1340,6 +1340,37 @@ static bool fuse_dio_wr_exclusive_lock(struct kiocb *iocb, struct iov_iter *from
 	return false;
 }
 
+static void fuse_dio_lock(struct kiocb *iocb, struct iov_iter *from,
+			  bool *exclusive)
+{
+	struct inode *inode = file_inode(iocb->ki_filp);
+
+	*exclusive = fuse_dio_wr_exclusive_lock(iocb, from);
+	if (*exclusive) {
+		inode_lock(inode);
+	} else {
+		inode_lock_shared(inode);
+		/*
+		 * Previous check was without inode lock and might have raced,
+		 * check again.
+		 */
+		if (fuse_io_past_eof(iocb, from)) {
+			inode_unlock_shared(inode);
+			inode_lock(inode);
+			*exclusive = true;
+		}
+	}
+}
+
+static void fuse_dio_unlock(struct inode *inode, bool exclusive)
+{
+	if (exclusive) {
+		inode_unlock(inode);
+	} else {
+		inode_unlock_shared(inode);
+	}
+}
+
 static ssize_t fuse_cache_write_iter(struct kiocb *iocb, struct iov_iter *from)
 {
 	struct file *file = iocb->ki_filp;
@@ -1604,30 +1635,9 @@ static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct inode *inode = file_inode(iocb->ki_filp);
 	struct fuse_io_priv io = FUSE_IO_PRIV_SYNC(iocb);
 	ssize_t res;
-	bool exclusive_lock = fuse_dio_wr_exclusive_lock(iocb, from);
-
-	/*
-	 * Take exclusive lock if
-	 * - Parallel direct writes are disabled - a user space decision
-	 * - Parallel direct writes are enabled and i_size is being extended.
-	 * - Shared mmap on direct_io file is supported (FUSE_DIRECT_IO_ALLOW_MMAP).
-	 *   This might not be needed at all, but needs further investigation.
-	 */
-	if (exclusive_lock)
-		inode_lock(inode);
-	else {
-		inode_lock_shared(inode);
-
-		/*
-		 * Previous check was without any lock and might have raced.
-		 */
-		if (fuse_io_past_eof(iocb, from)) {
-			inode_unlock_shared(inode);
-			inode_lock(inode);
-			exclusive_lock = true;
-		}
-	}
+	bool exclusive;
 
+	fuse_dio_lock(iocb, from, &exclusive);
 	res = generic_write_checks(iocb, from);
 	if (res > 0) {
 		if (!is_sync_kiocb(iocb) && iocb->ki_flags & IOCB_DIRECT) {
@@ -1638,10 +1648,7 @@ static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
 			fuse_write_update_attr(inode, iocb->ki_pos, res);
 		}
 	}
-	if (exclusive_lock)
-		inode_unlock(inode);
-	else
-		inode_unlock_shared(inode);
+	fuse_dio_unlock(inode, exclusive);
 
 	return res;
 }