[PATCH] deal with the first call of ->show() generating no output

seq_read() has a subtle bug - we want the first loop there to go
until at least one *non-empty* record had fit entirely into buffer.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/seq_file.c

@@ -108,9 +108,9 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 			goto Done;
 	}
 	/* we need at least one record in buffer */
+	pos = m->index;
+	p = m->op->start(m, &pos);
 	while (1) {
-		pos = m->index;
-		p = m->op->start(m, &pos);
 		err = PTR_ERR(p);
 		if (!p || IS_ERR(p))
 			break;
@@ -119,6 +119,11 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 			break;
 		if (unlikely(err))
 			m->count = 0;
+		if (unlikely(!m->count)) {
+			p = m->op->next(m, p, &pos);
+			m->index = pos;
+			continue;
+		}
 		if (m->count < m->size)
 			goto Fill;
 		m->op->stop(m, p);
@@ -128,6 +133,8 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
 			goto Enomem;
 		m->count = 0;
 		m->version = 0;
+		pos = m->index;
+		p = m->op->start(m, &pos);
 	}
 	m->op->stop(m, p);
 	m->count = 0;