On Tue, Nov 24, 2020 at 11:22:03AM +0800, Jason Wang wrote:

On 2020/11/24 上午3:21, Jakub Kicinski wrote:

quoted

On Mon, 23 Nov 2020 14:09:34 -0500 Steven Rostedt wrote:

quoted

On Mon, 23 Nov 2020 10:52:52 -0800
Jakub Kicinski [off-list ref] wrote:

quoted

On Mon, 23 Nov 2020 09:31:28 -0500 Steven Rostedt wrote:

quoted

On Mon, 23 Nov 2020 13:08:55 +0200
Leon Romanovsky [off-list ref] wrote:

quoted

  [   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()

quoted

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.

quoted

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

quoted