netdev - Re: netconsole deadlock with virtnet

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Date:   Tue, 24 Nov 2020 11:22:03 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Leon Romanovsky <leon@...nel.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 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

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

Thanks

>