netdev - Re: [Xen-devel] [PATCH net] xen-netback: add the scenario which now beyond the range time_after

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Date:	Thu, 17 Oct 2013 09:26:23 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Jason Luan" <jianhai.luan@...cle.com>
Cc:	<david.vrabel@...rix.com>, <ian.campbell@...rix.com>,
	<wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>,
	<annie.li@...cle.com>, <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH net] xen-netback: add the scenario
 which now beyond the range time_after_eq().

>>> On 16.10.13 at 19:22, Jason Luan <jianhai.luan@...cle.com> wrote:
> time_after_eq() only works if the delta is < MAX_ULONG/2.
> 
> If netfront sends at a very low rate, the time between subsequent calls
> to tx_credit_exceeded() may exceed MAX_ULONG/2 and the test for
> timer_after_eq() will be incorrect.  Credit will not be replenished and
> the guest may become unable to send (e.g., if prior to the long gap, all
> credit was exhausted).
> 
> We should add the scenario which now beyond next_credit+MAX_UNLONG/2. Because
> the fact now must be not before than expire, time_before(now, expire) == true
> will verify the scenario.
>     time_after_eq(now, next_credit) || time_before (now, expire)
>     ==
>     !time_in_range_open(now, expire, next_credit)

So first of all this must be with a 32-bit netback. And the not
coverable gap between activity is well over 240 days long. _If_
this really needs dealing with, then why is extending this from
240+ to 480+ days sufficient? I.e. why don't you simply
change to 64-bit jiffy values, and use time_after_eq64()?

Jan

> Signed-off-by: Jason Luan <jianhai.luan@...cle.com>
> ---
>  drivers/net/xen-netback/netback.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c 
> b/drivers/net/xen-netback/netback.c
> index f3e591c..31eedaf 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1194,8 +1194,11 @@ static bool tx_credit_exceeded(struct xenvif *vif, 
> unsigned size)
>  	if (timer_pending(&vif->credit_timeout))
>  		return true;
>  
> -	/* Passed the point where we can replenish credit? */
> -	if (time_after_eq(now, next_credit)) {
> +	/* Credit should be replenished when now does not fall into the
> +	 * range from expires to next_credit, and time_in_range_open()
> +	 * is used to verify whether this case happens.
> +	 */
> +	if (!time_in_range_open(now, vif->credit_timeout.expires, next_credit)) {
>  		vif->credit_timeout.expires = now;
>  		tx_add_credit(vif);
>  	}
> -- 
> 1.7.6.5
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@...ts.xen.org 
> http://lists.xen.org/xen-devel 



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