netdev - Re: netconsole deadlock with virtnet

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Date:   Tue, 24 Nov 2020 16:57:23 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Petr Mladek <pmladek@...e.com>,
        John Ogness <john.ogness@...utronix.de>,
        virtualization@...ts.linux-foundation.org,
        Amit Shah <amit@...nel.org>, Itay Aveksis <itayav@...dia.com>,
        Ran Rozenstein <ranro@...dia.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: netconsole deadlock with virtnet


On 2020/11/24 下午4:01, Leon Romanovsky wrote:
> On Tue, Nov 24, 2020 at 11:22:03AM +0800, Jason Wang wrote:
>> On 2020/11/24 上午3:21, Jakub Kicinski wrote:
>>> On Mon, 23 Nov 2020 14:09:34 -0500 Steven Rostedt wrote:
>>>> On Mon, 23 Nov 2020 10:52:52 -0800
>>>> Jakub Kicinski <kuba@...nel.org> wrote:
>>>>
>>>>> On Mon, 23 Nov 2020 09:31:28 -0500 Steven Rostedt wrote:
>>>>>> On Mon, 23 Nov 2020 13:08:55 +0200
>>>>>> Leon Romanovsky <leon@...nel.org> wrote:
>>>>>>
>>>>>>>    [   10.028024] Chain exists of:
>>>>>>>    [   10.028025]   console_owner --> target_list_lock --> _xmit_ETHER#2
>>>>>> Note, the problem is that we have a location that grabs the xmit_lock while
>>>>>> holding target_list_lock (and possibly console_owner).
>>>>> Well, it try_locks the xmit_lock. Does lockdep understand try-locks?
>>>>>
>>>>> (not that I condone the shenanigans that are going on here)
>>>> Does it?
>>>>
>>>> 	virtnet_poll_tx() {
>>>> 		__netif_tx_lock() {
>>>> 			spin_lock(&txq->_xmit_lock);
>>> Umpf. Right. I was looking at virtnet_poll_cleantx()
>>>
>>>> That looks like we can have:
>>>>
>>>>
>>>> 	CPU0		CPU1
>>>> 	----		----
>>>>      lock(xmit_lock)
>>>>
>>>> 		    lock(console)
>>>> 		    lock(target_list_lock)
>>>> 		    __netif_tx_lock()
>>>> 		        lock(xmit_lock);
>>>>
>>>> 			[BLOCKED]
>>>>
>>>>      <interrupt>
>>>>      lock(console)
>>>>
>>>>      [BLOCKED]
>>>>
>>>>
>>>>
>>>>    DEADLOCK.
>>>>
>>>>
>>>> So where is the trylock here?
>>>>
>>>> Perhaps you need the trylock in virtnet_poll_tx()?
>>> That could work. Best if we used normal lock if !!budget, and trylock
>>> when budget is 0. But maybe that's too hairy.
>>
>> If we use trylock, we probably lose(or delay) tx notification that may have
>> side effects to the stack.
>>
>>
>>> I'm assuming all this trickiness comes from virtqueue_get_buf() needing
>>> locking vs the TX path? It's pretty unusual for the completion path to
>>> need locking vs xmit path.
>>
>> Two reasons for doing this:
>>
>> 1) For some historical reason, we try to free transmitted tx packets in xmit
>> (see free_old_xmit_skbs() in start_xmit()), we can probably remove this if
>> we remove the non tx interrupt mode.
>> 2) virtio core requires virtqueue_get_buf() to be synchronized with
>> virtqueue_add(), we probably can solve this but it requires some non trivial
>> refactoring in the virtio core
> So how will we solve our lockdep issues?
>
> Thanks


It's not clear to me that whether it's a virtio-net specific issue. E.g 
the above deadlock looks like a generic issue so workaround it via 
virtio-net may not help for other drivers.

Thanks


>
>> Btw, have a quick search, there are several other drivers that uses tx lock
>> in the tx NAPI.
>>
>> Thanks
>>