linux-kernel - Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd

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Date:   Wed, 14 Jul 2021 10:10:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Jason Wang (jasowang@...hat.com)" <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

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