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

[Jason Wang]

在 2021/7/14 下午4:10, Paolo Bonzini 写道:
> On 14/07/21 10:01, Daniel Bristot de Oliveira wrote:
> > Hey
> >
> > I use kvm-vm for regular development, and while using the kernel-rt 
> > v5.13-rt1
> > (the latest) on the host, and a regular kernel on the guest, after a 
> > while,
> > this happens:
> >
> > [ 1723.404979] ------------[ cut here ]------------
> > [ 1723.404981] WARNING: CPU: 12 PID: 2554 at fs/eventfd.c:74 
> > eventfd_signal+0x7e/0x90
>
> > [ 1723.405055] RIP: 0010:eventfd_signal+0x7e/0x90
> > [ 1723.405059] Code: 01 00 00 00 be 03 00 00 00 4c 89 ef e8 5b ec d9 
> > ff 65 ff 0d e4 34 c9 5a 4c 89 ef e8 ec a8 86 00 4c 89 e0 5b 5d 41 5c 
> > 41 5d c3 <0f> 0b 45 31 e4 5b 5d 4c 89 e0 41 5c 41 5d c3 0f 1f 00 0f 
> > 1f 44 00
> > [ 1723.405078]  vhost_tx_batch.constprop.0+0x7d/0xc0 [vhost_net]
> > [ 1723.405083]  handle_tx_copy+0x15b/0x5c0 [vhost_net]
> > [ 1723.405088]  ? __vhost_add_used_n+0x200/0x200 [vhost]
> > [ 1723.405092]  handle_tx+0xa5/0xe0 [vhost_net]
> > [ 1723.405095]  vhost_worker+0x93/0xd0 [vhost]
> > [ 1723.405099]  kthread+0x186/0x1a0
> > [ 1723.405103]  ? __kthread_parkme+0xa0/0xa0
> > [ 1723.405105]  ret_from_fork+0x22/0x30
> > [ 1723.405110] ---[ end trace 0000000000000002 ]---
>
> The WARN has this comment above:
>
>         /*
>          * Deadlock or stack overflow issues can happen if we recurse 
> here
>          * through waitqueue wakeup handlers. If the caller users 
> potentially
>          * nested waitqueues with custom wakeup handlers, then it should
>          * check eventfd_signal_count() before calling this function. If
>          * it returns true, the eventfd_signal() call should be 
> deferred to a
>          * safe context.
>          */
>
> This was added in 2020, so it's unlikely to be the direct cause of the
> change.  What is a known-good version for the host?
>
> Since it is not KVM stuff, I'm CCing Michael and Jason.
>
> Paolo

I think this can be probably fixed here:

https://lore.kernel.org/lkml/20210618084412.18257-1-zhe.he@xxxxxxxxxxxxx/

Thanks