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

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Message-Id: <511CFF8A02000078000BE4B2@nat28.tlf.novell.com>
Date:	Thu, 14 Feb 2013 14:15:22 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ian Campbell" <Ian.Campbell@...rix.com>,
	"Wei Liu" <wei.liu2@...rix.com>
Cc:	"David Vrabel" <david.vrabel@...rix.com>,
	"Wei Liu (3P)" <wei.liu@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"KonradRzeszutek Wilk" <konrad.wilk@...cle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 2/2] xen-netback: cancel the credit
 timer when taking the vif down

>>> On 14.02.13 at 14:53, Wei Liu <wei.liu2@...rix.com> 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`.

Which gets us to another aspect of the original fix that I don't
think was considered: Is there anything preventing the interface
to be brought back up after fatal_tx_err() shut it down?

Jan

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