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Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
To:     Jason Wang <jasowang@redhat.com>,
        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>,
        Juri Lelli <jlelli@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Xie Yongji <xieyongji@bytedance.com>
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From:   He Zhe <zhe.he@windriver.com>
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Date:   Fri, 16 Jul 2021 10:43:07 +0800
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On 7/16/21 10:26 AM, Jason Wang wrote:
>
> 在 2021/7/15 下午7:05, Paolo Bonzini 写道:
>> 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.
>
>
> For VDUSE we may still need the patch since it tries to relay notifications (eventfds) which means the recursion of the eventfd signal.
>
> But looking at the comment in the eventfd_signal() which say we should check with eventfd_signal_count() and delay the signal into a safe context (e.g workqueue etc).

The main concern when adding eventfd count at the very beginning is "Deadlock or stack overflow" in the inline comment. If we can avoid deadlock and one depth of nest is acceptable, I think it's safe to set the max count to 2.

The author of the eventfd count kind of also agrees with this.
https://lore.kernel.org/lkml/3b4aa4cb-0e76-89c2-c48a-cf24e1a36bc2@kernel.dk/

Thanks,
Zhe

>
> Thanks
>
>
>>
>> Paolo
>>
>