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 <k...@kernel.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 <l...@kernel.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
>
> >
>
