netdev - Re: netconsole deadlock with virtnet

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Date:   Tue, 24 Nov 2020 10:01:52 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Jason Wang <jasowang@...hat.com>
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 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

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