netdev - Re: netconsole deadlock with virtnet

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Date:   Mon, 23 Nov 2020 14:09:34 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jakub Kicinski <kuba@...nel.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 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);

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()?

-- Steve