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

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Message-ID: <c05b358b-1e96-c002-085c-b25e416e7be5@redhat.com>
Date:   Thu, 15 Jul 2021 13:05:57 +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 12:10, He Zhe wrote:
> The following was provided in this thread. The commit log contains the call traces that I met and fixed back to Apr. 2020.
> 
> https://lore.kernel.org/lkml/20210618084412.18257-1-zhe.he@windriver.com/

> 001: WARNING: CPU: 1 PID: 1503 at fs/eventfd.c:73 eventfd_signal+0x85/0xa0
> ---- snip ----
> 001: Call Trace:
> 001:  vhost_signal+0x15e/0x1b0 [vhost]
> 001:  vhost_add_used_and_signal_n+0x2b/0x40 [vhost]
> 001:  handle_rx+0xb9/0x900 [vhost_net]
> 001:  handle_rx_net+0x15/0x20 [vhost_net]
> 001:  vhost_worker+0xbe/0x120 [vhost]
> 001:  kthread+0x106/0x140
> 001:  ? log_used.part.0+0x20/0x20 [vhost]
> 001:  ? kthread_park+0x90/0x90
> 001:  ret_from_fork+0x35/0x40

This call trace is not of a reentrant call; there is only one call to 
eventfd_signal.  It does fit the symptoms that Daniel reported for 
PREEMPT_RT though.

> https://lore.kernel.org/lkml/beac2025-2e11-8ed0-61e2-9f6e633482e8@redhat.com/

This one is about PREEMPT_RT, so it would be fixed by local_lock.

There _may_ be two bugs, so let's start by fixing this one.  Once this 
one is fixed, we will examine the call stacks of any further reports, 
and diagnose whether the second bug (if it exists) is related to vDUSE, 
PREEMPT_RT or neeither.

Paolo