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

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Message-ID: <a2f3f9ac-dac2-eadc-269e-91652d78ebd3@redhat.com>
Date:   Fri, 16 Jul 2021 13:55:27 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Juri Lelli <jlelli@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        He Zhe <zhe.he@...driver.com>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

On 16/07/21 11:37, Hillf Danton wrote:
> On Fri, 16 Jul 2021 09:59:15 +0200 Paolo Bonzini wrote:
>> * the warning only occurs if preemption occurs during the
>> spin_lock_irqsave critical section (and therefore it can only occur in
>> PREEMPT_RT kernels)
> 
> With that lock held, no waitqueue entry can be added on to the WQ - IOW no
> wakeup will go stray.
> 
>> * the warning causes an early return 0 that messes up the VM's networking
> 
> Is the messup due to the zero or wakeup?

It's caused by the missing wakeup, i.e. eventfd_signal not really 
signaling anything.

Paolo