Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

[Paolo Bonzini]

On 16/07/21 09:55, Hillf Danton wrote:
> On Fri, 16 Jul 2021 08:54:58 +0200 Paolo Bonzini wrote:
> > On 16/07/21 04:06, Hillf Danton wrote:
> > > > With the patch:
> > > > 	- no warn
> > > > 	- continue using the VM normally...
> > > Well with the patch applied, the VM works fine without the stuff protected
> > > by the spin_lock_irqsave(), then without the patch why simply printing a
> > > warning makes the VM dumb, given the warning is there actually also
> > > preventing you from touching the lock.
> >
> > If the warning is triggered, eventfd_signal will not do the wakeup.
>
> [I am assuming we are not talking about the deadlock in the comment.]
>
> No preemption occured without the warning printed.
> Why will the wakeup behavior change without peemption?

Sorry, I don't follow.  What I'm saying is that without the patch:

* the warning only occurs if preemption occurs during the 
spin_lock_irqsave critical section (and therefore it can only occur in 
PREEMPT_RT kernels)

* the warning causes an early return 0 that messes up the VM's networking

Paolo