netdev - Re: [Xen-devel] [PATCH 2/2] xen-netback: cancel the credit timer when taking the vif down

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Date:	Thu, 14 Feb 2013 13:56:24 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Jan Beulich <jbeulich@...e.com>,
	"Wei Liu (3P)" <wei.liu@...rix.com>
Subject: Re: [Xen-devel] [PATCH 2/2] xen-netback: cancel the credit timer
 when taking the vif down

On 14/02/13 13:53, Wei Liu wrote:
> On Thu, 2013-02-14 at 13:18 +0000, David Vrabel wrote:
>> From: David Vrabel <david.vrabel@...rix.com>
>>
>> If the credit timer is left armed after calling
>> xen_netbk_remove_xenvif(), then it may fire and attempt to schedule
>> the vif which will then oops as vif->netbk == NULL.
>>
>> This may happen both in the fatal error path and during normal
>> disconnection from the front end.
>>
>> The sequencing during shutdown is critical to ensure that: a)
>> vif->netbk doesn't become unexpectedly NULL; and b) the net device/vif
>> is not freed.
>>
>> 1. Mark as unschedulable (netif_carrier_off()).
>> 2. Synchronously cancel the timer.
>> 3. Remove the vif from the schedule list.
>> 4. Remove it from it netback thread group.
>> 5. Wait for vif->refcnt to become 0.
>>
>> Signed-off-by: David Vrabel <david.vrabel@...rix.com>
> 
> You would need to reinitialize the timer in xenvif_up, given that user
> might `ifconfig vifX.X down; ifconfig vifX.X up`.

No.  Deleted timers do not need to be reinitialized.  The timer will be
armed as usual with mod_timer() when credit is next exhausted.

David
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