netdev - Re: rose timer t error displayed in /proc/net/rose

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Date:   Mon, 1 Aug 2022 17:44:33 +0200
From:   Francois Romieu <romieu@...zoreil.com>
To:     Thomas Osterried <thomas@...erried.de>
Cc:     Bernard f6bvp <f6bvp@...e.fr>, Eric Dumazet <edumazet@...gle.com>,
        linux-hams@...r.kernel.org,
        Thomas Osterried DL9SAU <thomas@...erg.in-berlin.de>,
        netdev@...r.kernel.org
Subject: Re: rose timer t error displayed in /proc/net/rose

Thomas Osterried <thomas@...erried.de> :
[...]
> 1. why do you check for pending timer anymore?

s/do/don't/ ?

I don't check for pending timer because:
- the check is racy
- jiffies_delta_to_clock_t maxes negative delta to zero

> 2. I'm not really sure what value jiffies_delta_to_clock_t() returns. jiffies / HZ?
>    jiffies_delta_to_clock_t() returns clock_t.

I completely messed this part :o/

clock_t relates to USER_HZ like jiffies does to HZ.

[don't break the ax25]

Sure, we are in violent agreement here.

[...]
> 1. I expect rose->timer to be restarted soon. If it does not happen, is there a bug?

The relevant rose state in Bernard's sample was ROSE_STATE_2.

net/rose/rose_timer.c::rose_timer_expiry would had called
rose_disconnect(sk, ETIMEDOUT, -1, -1) so there should not
be any timer restart (afaiu, etc.).

[...]
>    => Negative may be handled due to Francois' patch now correctly.
> delta as signed long may be negative. max(0L, -nnnn) sould result to 0L.
>    This would result to 0. Perhaps proven by Francois, because he used this function and achieved a correct display of that idle value. Francois, am I correct, is "0" really displayed?

I must confess that I was not terribly professional this morning past 2AM.

The attached snippet illustrates the behavior wrt negative values
(make; insmod foo.ko ; sleep 1; rmmod foo.ko; dmesg | tail -n 2).
It also illustrates that I got the unit wrong.

This should be better:

diff --git a/net/ax25/ax25_timer.c b/net/ax25/ax25_timer.c
index 85865ebfdfa2..3c94e5a2d098 100644
--- a/net/ax25/ax25_timer.c
+++ b/net/ax25/ax25_timer.c
@@ -108,10 +108,9 @@ int ax25_t1timer_running(ax25_cb *ax25)
 
 unsigned long ax25_display_timer(struct timer_list *timer)
 {
-	if (!timer_pending(timer))
-		return 0;
+	long delta = timer->expires - jiffies;
 
-	return timer->expires - jiffies;
+	return max(0L, delta);
 }
 
 EXPORT_SYMBOL(ax25_display_timer);


View attachment "Makefile" of type "text/plain" (129 bytes)

View attachment "foo.c" of type "text/plain" (513 bytes)