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

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Date:   Thu, 15 Jul 2021 11:51:32 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     He Zhe <zhe.he@...driver.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        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>
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 15/07/21 10:44, He Zhe wrote:
> It was to increase the recursion depth to 2 so that vhost_worker and
> kvm_vcpu_ioctl syscall could work in parallel

The count is per-CPU, so parallel operations cannot cause it to become 
2.  Your patch might fix calls from ioeventfd to vhost_worker to another 
eventfd, but not *parallel* operation of KVM and vhost (except on 
PREEMPT_RT).

You should identify the exact callstack that caused the warning for 
vDUSE, and document that one in the commit message, so that reviewers 
can understand the issue.

Paolo