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

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Date:   Mon, 26 Jul 2021 13:03:15 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Michael S. Tsirkin" <mst@...hat.com>, linux-mm@...ck.org,
        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 24/07/21 06:33, Hillf Danton wrote:
> 		lock WQ
> 		count += n
> 		no waiter
> 		unlock WQ

Ok, this is a write.

> 
> 				lock WQ
> 				add waiter for EPOLLIN
> 				unlock WQ

This is eventfd_poll().  It hasn't yet returned EPOLLIN.

> 						lock WQ
> 						count = 0
> 						wakeup EPOLLOUT
> 						unlock WQ

This is a read().

> lock WQ
> count += n
> no waiter
> unlock WQ

This is wrong; after "unlock WQ" in CPU3 there *is* a waiter, no one has 
waked it up yet.

Paolo

> ------------------------------- c1 = count