netdev - Re: [PATCH 2/3] Revert "e1000e: Separate signaling for link check/link up"

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Date:   Mon, 29 Jan 2018 07:48:26 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Benjamin Poirier <bpoirier@...e.com>
Cc:     Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        Netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] Revert "e1000e: Separate signaling for link
 check/link up"

On Sun, Jan 28, 2018 at 11:21 PM, Benjamin Poirier <bpoirier@...e.com> wrote:
> On 2018/01/26 09:03, Alexander Duyck wrote:
>> On Fri, Jan 26, 2018 at 1:12 AM, Benjamin Poirier <bpoirier@...e.com> wrote:
>> > This reverts commit 19110cfbb34d4af0cdfe14cd243f3b09dc95b013.
>> > This reverts commit 4110e02eb45ea447ec6f5459c9934de0a273fb91.
>> >
>> > ... because they cause an extra 2s delay for the link to come up when
>> > autoneg is off.
>> >
>> > After reverting, the race condition described in the log of commit
>> > 19110cfbb34d ("e1000e: Separate signaling for link check/link up") is
>> > reintroduced. It may still be triggered by LSC events but this should not
>> > result in link flap. It may no longer be triggered by RXO events because
>> > commit 4aea7a5c5e94 ("e1000e: Avoid receiver overrun interrupt bursts")
>> > restored reading icr in the Other handler.
>>
>> With the RXO events removed the only cause for us to transition the
>> bit should be LSC. I'm not sure if the race condition in that state is
>> a valid concern or not as the LSC should only get triggered if the
>> link state toggled, even briefly.
>>
>> The bigger concern I would have would be the opposite of the original
>> race that was pointed out:
>>     \ e1000_watchdog_task
>>         \ e1000e_has_link
>>             \ hw->mac.ops.check_for_link() === e1000e_check_for_copper_link
>>                 /* link is up */
>>                 mac->get_link_status = false;
>>
>>                                 /* interrupt */
>>                                 \ e1000_msix_other
>>                                     hw->mac.get_link_status = true;
>>
>>             link_active = !hw->mac.get_link_status
>>             /* link_active is false, wrongly */

     \ e1000_watchdog_task
         \ e1000e_has_link
             \ hw->mac.ops.check_for_link() === e1000e_check_for_copper_link
                                 /* interrupt */
                                 \ e1000_msix_other
                                     hw->mac.get_link_status = true;

                /* link is up */
                 mac->get_link_status = false;

             link_active = !hw->mac.get_link_status
             /* link_active is true, wrongly */

So basically the idea would be that the interrupt fires after we check
for link, but before we have updated get_link_status. It should be a
pretty narrow window and I don't know if it will be much of an issue.

>> So the question I would have is what if we see the LSC for a link down
>> just after the check_for_copper_link call completes? It may not be
>
> Can you write out exactly what that race would be, in a format similar to the
> above?

I just did the copy/paste/edit above if that works. Hopefully that
makes it clearer.

>> anything seen in the real world since I don't know if we have any link
>> flapping issues on e1000e or not without this patch. It is something
>> to keep in mind for the future though.
>>
>>
>> > As discussed, the driver should be in "maintenance mode". In the interest
>> > of stability, revert to the original code as much as possible instead of a
>> > half-baked solution.
>>
>> If nothing else we may want to do a follow-up on this patch as we
>> probably shouldn't be returning the error values to trigger link up.
>> There are definitely issues to be found here. If nothing else we may
>> want to explore just returning 1 if auto-neg is disabled instead of
>> returning an error code.
>>
>> > Link: https://www.spinics.net/lists/netdev/msg479923.html
>> > Signed-off-by: Benjamin Poirier <bpoirier@...e.com>
> [...]