netdev - Re: [PATCH] sctp: don't compare hb_timer expire date before starting it

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Date:   Fri, 22 Feb 2019 10:03:39 +0000
From:   "Sverdlin, Alexander (Nokia - DE/Ulm)" <alexander.sverdlin@...ia.com>
To:     "kwiecienmaciek@...il.com" <kwiecienmaciek@...il.com>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Kwiecien, Maciej (Nokia - PL/Wroclaw)" <maciej.kwiecien@...ia.com>
Subject: Re: [PATCH] sctp: don't compare hb_timer expire date before starting
 it

On 22/02/2019 09:45, kwiecienmaciek@...il.com wrote:
> From: Maciej Kwiecien <maciej.kwiecien@...ia.com>
> 
> hb_timer might not start at all for a particular transport because its
> start is conditional. In a result a node is not sending heartbeats.
> 
> Function sctp_transport_reset_hb_timer has two roles:
>     - initial start of hb_timer for a given transport,
>     - update expire date of hb_timer for a given transport.
> The function is optimized to update timer's expire only if it is before
> a new calculated one but this comparison is invalid for a timer which
> has not yet started. Such a timer has expire == 0 and if a new expire
> value is bigger than (MAX_JIFFIES / 2 + 2) then "time_before" macro will
> fail and timer will not start resulting in no heartbeat packets send by
> the node.
> 
> This was found when association was initialized within first 5 mins
> after system boot due to jiffies init value which is near to MAX_JIFFIES.
> 
> Test kernel version: 4.9.154 (ARCH=arm)
> hb_timer.expire = 0;                //initialized, not started timer
> new_expire = MAX_JIFFIES / 2 + 2;   //or more
> time_before(hb_timer.expire, new_expire) == false
> 
> Fixes: ba6f5e33bdbb ("sctp: avoid refreshing heartbeat timer too often")
> Reported-by: Marcin Stojek <marcin.stojek@...ia.com>
> Tested-by: Marcin Stojek <marcin.stojek@...ia.com>

Reviewed-by: Alexander Sverdlin <alexander.sverdlin@...ia.com>

> Signed-off-by: Maciej Kwiecien <maciej.kwiecien@...ia.com>
> ---
>  net/sctp/transport.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/sctp/transport.c b/net/sctp/transport.c
> index 033696e6f74f..ad158d311ffa 100644
> --- a/net/sctp/transport.c
> +++ b/net/sctp/transport.c
> @@ -207,7 +207,8 @@ void sctp_transport_reset_hb_timer(struct sctp_transport *transport)
>  
>  	/* When a data chunk is sent, reset the heartbeat interval.  */
>  	expires = jiffies + sctp_transport_timeout(transport);
> -	if (time_before(transport->hb_timer.expires, expires) &&
> +	if ((time_before(transport->hb_timer.expires, expires) ||
> +	     !timer_pending(&transport->hb_timer)) &&
>  	    !mod_timer(&transport->hb_timer,
>  		       expires + prandom_u32_max(transport->rto)))
>  		sctp_transport_hold(transport);

-- 
Best regards,
Alexander Sverdlin.