[PATCH] timer_list: Correct the show function for timer_list by using iter->now

[PATCH] timer_list: Correct the show function for timer_list by using iter->now

[Nathan Zimmer]

This patch corrects the issue with /proc/timer_list reported by Holger.
When reading from the proc file with a sufficently small buffer, 2k so not
really that small, there was one could get hung trying to read the file a
chunk at a time.

Signed-off-by Nathan Zimmer <nzimmer@sgi.com>
Reported-by: Holger Hans Peter Freyther <holger@freyther.de>
Cc: <stable@vger.kernel.org> # 3.10.x
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/time/timer_list.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 3bdf283..08d7fbd 100644
--- a/kernel/time/timer_list.c
+++ b/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
-- 
1.8.2.1

Re: [PATCH] timer_list: Correct the show function for timer_list by using iter->now

[Holger Hans Peter Freyther]

On Mon, Jul 22, 2013 at 04:18:31PM -0500, Nathan Zimmer wrote:
> This patch corrects the issue with /proc/timer_list reported by Holger.
> When reading from the proc file with a sufficently small buffer, 2k so not
> really that small, there was one could get hung trying to read the file a
> chunk at a time.

I think it makes sense to always use iter->now but it does not solve
the issue with dropbear/my test case. Or is this meant to be applied
on top of the previous patch?


holger

Re: [PATCH] timer_list: Correct the show function for timer_list by using iter->now

[Nathan Zimmer]

On 07/23/2013 02:18 AM, Holger Hans Peter Freyther wrote:
> On Mon, Jul 22, 2013 at 04:18:31PM -0500, Nathan Zimmer wrote:
>> This patch corrects the issue with /proc/timer_list reported by Holger.
>> When reading from the proc file with a sufficently small buffer, 2k so not
>> really that small, there was one could get hung trying to read the file a
>> chunk at a time.
> I think it makes sense to always use iter->now but it does not solve
> the issue with dropbear/my test case. Or is this meant to be applied
> on top of the previous patch?
>
>
> holger
>
Ah sorry I networked booted the wrong kernel and got excited.


However after hitting my head against this for the better part of the 
day I found
the root cause of my issue.

In seq_read about halfway down when we flush the buffer to userland.
m->index is advanced without calling m->op->next().
My iterator doesn't take notice of that and thus fails to advance.

         /* if not empty - flush it first */
         if (m->count) {
                 n = min(m->count, size);
                 err = copy_to_user(buf, m->buf + m->from, n);
                 if (err)
                         goto Efault;
                 m->count -= n;
                 m->from += n;
                 size -= n;
                 buf += n;
                 copied += n;
                 if (!m->count)
                         m->index++;
....


I'll need to think about how to correct it.

For the moment probably the right thing would be to revert b3956a896ea5

[PATCH] timer_list: Correct the iterator for timer_list

[Nathan Zimmer]

This patch corrects the 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: <stable@vger.kernel.org> # 3.10.x
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 kernel/time/timer_list.c | 41 ++++++++++++++++++++++++-----------------
 1 file changed, 24 insertions(+), 17 deletions(-)

diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 3bdf283..61ed862 100644
--- a/kernel/time/timer_list.c
+++ b/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 *timer_list_start(struct seq_file *file, loff_t *offset)
+static void *move_iter(struct timer_list_iter *iter, loff_t offset)
 {
-	struct timer_list_iter *iter = file->private;
-
-	if (!*offset) {
-		iter->cpu = -1;
-		iter->now = ktime_to_ns(ktime_get());
-	} else if (iter->cpu >= nr_cpu_ids) {
+	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;
+			if (!iter->second_pass) {
+				iter->cpu = -1;
+				iter->second_pass = true;
+			} else
+				return NULL;
 #else
-		return NULL;
+			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->now = ktime_to_ns(ktime_get());
+	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)
-- 
1.8.2.1