Re: 6.12-rc1: Lockdep regression bissected (virtio-net/console/scheduler)

[Breno Leitao]

On Wed, Oct 09, 2024 at 04:44:24PM +0100, Pavel Begunkov wrote:
> On 10/8/24 16:18, John Ogness wrote:
> > On 2024-10-04, Petr Mladek <pmladek@xxxxxxxx> wrote:
> > > On Fri 2024-10-04 02:08:52, Breno Leitao wrote:
> > > > 	 =====================================================
> > > > 	 WARNING: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
> > > > 	 6.12.0-rc1-kbuilder-virtme-00033-gd4ac164bde7a #50 Not tainted
> > > > 	 -----------------------------------------------------
> > > > 	 swapper/0/1 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
> > > > 	 ff1100010a260518 (_xmit_ETHER#2){+.-.}-{2:2}, at: virtnet_poll_tx (./include/linux/netdevice.h:4361 drivers/net/virtio_net.c:2969)
> > > > 
> > > > 	and this task is already holding:
> > > > 	 ffffffff86f2b5b8 (target_list_lock){....}-{2:2}, at: write_ext_msg (drivers/net/netconsole.c:?)
> > > > 	 which would create a new lock dependency:
> > > > 	  (target_list_lock){....}-{2:2} -> (_xmit_ETHER#2){+.-.}-{2:2}
> > > > 
> > > > 	but this new dependency connects a HARDIRQ-irq-safe lock:
> > > > 	  (console_owner){-...}-{0:0}
> > 
> > ...
> > 
> > > > 	to a HARDIRQ-irq-unsafe lock:
> > > > 	  (_xmit_ETHER#2){+.-.}-{2:2}
> > 
> > ...
> > 
> > > > 	other info that might help us debug this:
> > > > 
> > > > 	 Chain exists of:
> > > > 	console_owner --> target_list_lock --> _xmit_ETHER#2
> > > > 
> > > > 	  Possible interrupt unsafe locking scenario:
> > > > 
> > > > 		CPU0                    CPU1
> > > > 		----                    ----
> > > > 	   lock(_xmit_ETHER#2);
> > > > 					local_irq_disable();
> > > > 					lock(console_owner);
> > > > 					lock(target_list_lock);
> > > > 	   <Interrupt>
> > > > 	     lock(console_owner);
> > 
> > I can trigger this lockdep splat on v6.11 as well.
> > 
> > It only requires a printk() call within any interrupt handler, sometime
> > after the netconsole is initialized and has had at least one run from
> > softirq context.
> > 
> > > My understanding is that the fix is to always take "_xmit_ETHER#2"
> > > lock with interrupts disabled.
> > 
> > That seems to be one possible solution. But maybe there is reasoning why
> > that should not be done. (??) Right now it is clearly a spinlock that is
> 
> It's expensive, and it's a hot path if I understand correctly which
> lock that is. And, IIRC the driver might spend there some time, it's
> always nicer to keep irqs enabled if possible.

This also seems a broad network lock, which might have so many other
impacts beyond performance.

That said, I am running out of ideas on how to get this fixed,
unfortunately.

--breno