Re: "Dead loop on virtual device" error without softirq-BKL on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-02-18 13:50:14 [+0100], Bert Karwatzki wrote:
> Am Mittwoch, dem 18.02.2026 um 08:30 +0100 schrieb Sebastian Andrzej Siewior:
> > On 2026-02-17 20:10:09 [+0100], Bert Karwatzki wrote:
> > > 
> > > I tried to research the original commit which introduced the xmit_lock_owner check, but
> > > it is present since linux 2.3.6 (released 19990610) (when __dev_queue_xmit() was still called dev_queue_xmit()),
> > > so I can't tell the original idea behind that check (perhaps recuesion detection ...), so I'm
> > > not completely sure if it can be omitted (and just let dev_xmit_recursion() do the recursion checking).
> > 
> > Okay. Thank you. I add it to my list.
> > 
> I've thought about it again and I now think the xmit_lock_owner check IS necessary to
> avoid deadlocks on recursion in the non-RT case.
> 
> My idea to use get_current()->tgid as lock owner also does not work as interrupts are still enabled
> and __dev_queue_xmit() can be called from interrupt context. So in a situation where an interrupt occurs
> after the lock has been taken and the interrupt handler calls __dev_queue_xmit() on the same CPU a deadlock
> would occur.

The warning happens because taskA on cpuX goes through
HARD_TX_LOCK(), gets preempted and then taskB on cpuX wants also to send
send a packet. The second one throws the warning.

We could ignore this check because a deadlock will throw a warning and
"halt" the task that runs into the deadlock.
But then we could be smart about this in the same way !RT is and
pro-active check for the simple deadlock. The lock owner of
netdev_queue::_xmit_lock is recorded can be checked vs current.

The snippet below should work. I need to see if tomorrow this is still a
good idea.

diff --git a/net/core/dev.c b/net/core/dev.c
index 6ff4256700e60..de342ceb17201 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4821,7 +4821,7 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
 		/* Other cpus might concurrently change txq->xmit_lock_owner
 		 * to -1 or to their cpu id, but not to our id.
 		 */
-		if (READ_ONCE(txq->xmit_lock_owner) != cpu) {
+		if (rt_mutex_owner(&txq->_xmit_lock.lock) != current) {
 			if (dev_xmit_recursion())
 				goto recursion_alert;
 

> Bert Karwatzki

Sebastian