netdev - Re: [PATCH net 1/1] ionic: wait on queue start until after IFF

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Date:   Tue, 9 Jun 2020 13:11:28 -0700
From:   Shannon Nelson <snelson@...sando.io>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net 1/1] ionic: wait on queue start until after IFF_UP

On 6/9/20 1:06 PM, David Miller wrote:
> From: Shannon Nelson <snelson@...sando.io>
> Date: Tue, 9 Jun 2020 12:51:17 -0700
>
>> On 6/9/20 12:47 PM, David Miller wrote:
>>> From: Shannon Nelson <snelson@...sando.io>
>>> Date: Mon,  8 Jun 2020 20:41:43 -0700
>>>
>>>> The netif_running() test looks at __LINK_STATE_START which
>>>> gets set before ndo_open() is called, there is a window of
>>>> time between that and when the queues are actually ready to
>>>> be run.  If ionic_check_link_status() notices that the link is
>>>> up very soon after netif_running() becomes true, it might try
>>>> to run the queues before they are ready, causing all manner of
>>>> potential issues.  Since the netdev->flags IFF_UP isn't set
>>>> until after ndo_open() returns, we can wait for that before
>>>> we allow ionic_check_link_status() to start the queues.
>>>>
>>>> On the way back to close, __LINK_STATE_START is cleared before
>>>> calling ndo_stop(), and IFF_UP is cleared after.  Both of
>>>> these need to be true in order to safely stop the queues
>>>> from ionic_check_link_status().
>>>>
>>>> Fixes: 49d3b493673a ("ionic: disable the queues on link down")
>>>> Signed-off-by: Shannon Nelson <snelson@...sando.io>
>>> What will make sure the queues actually get started if this
>>> event's queue start gets skipped in this scenerio?
>>>
>>> This code is only invoked when the link status changes or
>>> when the firmware is started.
>> If the link is already seen as up before ionic_open() is called it
>> starts the queues at that point.
>>
>> If link isn't seen until after ionic_open(), then the queues are
>> started in this periodic link check.
> I'm saying if it happens in the race condition you mention that you
> are protecting against, where running is true and IFF_UP is not.
>
> The link check is periodic?  It looks like it triggers when the
> link state changes to me.

Yes, the link check is triggered by the periodic watchdog 
ionic_watchdog_cb(), every 5 seconds.

sln