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

[John Ogness <john.ogness@linutronix.de>]

On 2024-10-04, Petr Mladek <pmladek@suse.com> 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
being taken from both interrupt and softirq contexts and does not
disable interrupts.

I will check if there is some previous kernel release where this problem
does not exist.

John Ogness