netdev - Re: netconsole deadlock with virtnet

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Date:   Mon, 23 Nov 2020 11:21:30 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Leon Romanovsky <leon@...nel.org>,
        Jason Wang <jasowang@...hat.com>,
        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 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.

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.