timer_list: correct the iterator for timer_list

Correct an issue with /proc/timer_list reported by Holger.

When reading from the proc file with a sufficiently small buffer, 2k so
not really that small, there was one could get hung trying to read the
file a chunk at a time.

The timer_list_start function failed to account for the possibility that
the offset was adjusted outside the timer_list_next.

Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
Reported-by: Holger Hans Peter Freyther <holger@freyther.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Berke Durak <berke.durak@xiphos.com>
Cc: Jeff Layton <jlayton@redhat.com>
Tested-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org> # 3.10.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/time/timer_list.c

@@ -265,10 +265,9 @@ static inline void timer_list_header(struct seq_file *m, u64 now)
 static int timer_list_show(struct seq_file *m, void *v)
 {
 	struct timer_list_iter *iter = v;
-	u64 now = ktime_to_ns(ktime_get());
 
 	if (iter->cpu == -1 && !iter->second_pass)
-		timer_list_header(m, now);
+		timer_list_header(m, iter->now);
 	else if (!iter->second_pass)
 		print_cpu(m, iter->cpu, iter->now);
 #ifdef CONFIG_GENERIC_CLOCKEVENTS
@@ -298,33 +297,41 @@ void sysrq_timer_list_show(void)
 	return;
 }
 
+static void *move_iter(struct timer_list_iter *iter, loff_t offset)
+{
+	for (; offset; offset--) {
+		iter->cpu = cpumask_next(iter->cpu, cpu_online_mask);
+		if (iter->cpu >= nr_cpu_ids) {
+#ifdef CONFIG_GENERIC_CLOCKEVENTS
+			if (!iter->second_pass) {
+				iter->cpu = -1;
+				iter->second_pass = true;
+			} else
+				return NULL;
+#else
+			return NULL;
+#endif
+		}
+	}
+	return iter;
+}
+
 static void *timer_list_start(struct seq_file *file, loff_t *offset)
 {
 	struct timer_list_iter *iter = file->private;
 
-	if (!*offset) {
-		iter->cpu = -1;
+	if (!*offset)
 		iter->now = ktime_to_ns(ktime_get());
-	} else if (iter->cpu >= nr_cpu_ids) {
-#ifdef CONFIG_GENERIC_CLOCKEVENTS
-		if (!iter->second_pass) {
-			iter->cpu = -1;
-			iter->second_pass = true;
-		} else
-			return NULL;
-#else
-		return NULL;
-#endif
-	}
-	return iter;
+	iter->cpu = -1;
+	iter->second_pass = false;
+	return move_iter(iter, *offset);
 }
 
 static void *timer_list_next(struct seq_file *file, void *v, loff_t *offset)
 {
 	struct timer_list_iter *iter = file->private;
-	iter->cpu = cpumask_next(iter->cpu, cpu_online_mask);
 	++*offset;
-	return timer_list_start(file, offset);
+	return move_iter(iter, 1);
 }
 
 static void timer_list_stop(struct seq_file *seq, void *v)