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Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        He Zhe <zhe.he@windriver.com>
References: <df278db6-1fc0-3d42-9c0e-f5a085c6351e@redhat.com>
 <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <f0254740-944d-201b-9a66-9db1fe480ca6@redhat.com>
Date:   Wed, 14 Jul 2021 17:23:02 +0800
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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@windriver.com/

Thanks