From: Paolo Bonzini <pbonzini@redhat.com>
To: Hillf Danton <hdanton@sina.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
"Michael S. Tsirkin" <mst@redhat.com>,
linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Date: Mon, 19 Jul 2021 17:38:45 +0200 [thread overview]
Message-ID: <724e7951-59c8-8b2b-37b8-6b0bf696ab04@redhat.com> (raw)
In-Reply-To: <20210718124219.1521-1-hdanton@sina.com>
On 18/07/21 14:42, Hillf Danton wrote:
>> It's caused by the missing wakeup, i.e. eventfd_signal not really
>> signaling anything.
>
> Can you please point me to the waiters in the mainline?
It's irqfd_wakeup.
> There are two cases of write_seqcount_begin in x/virt/kvm/eventfd.c, and
> in kvm_irqfd_deassign() it is surrounded by spin_lock_irq(&kvm->irqfds.lock)
> that also protects irqfd_update().
>
> What isnt clear is if the risk is zero that either case can be preempted by
> seqcount reader. That risk may end up with the livelock described in
> x/Documentation/locking/seqlock.rst.
Since the introduction of seqcount_spinlock_t, the writers automatically
disable preemption. This is definitely the right thing in this case
where the seqcount writers are small enough, and the readers are hot
enough, that using a local lock would be too heavyweight.
Without that, the livelock would be possible, though very unlikely. In
practice seqcount updates should only happen while the producer is
quiescent; and also the seqcount readers and writers will often be
pinned to separate CPUs.
Paolo
> +A sequence counter write side critical section must never be preempted
> +or interrupted by read side sections. Otherwise the reader will spin for
> +the entire scheduler tick due to the odd sequence count value and the
> +interrupted writer. If that reader belongs to a real-time scheduling
> +class, it can spin forever and the kernel will livelock.
>
next prev parent reply other threads:[~2021-07-19 15:38 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <df278db6-1fc0-3d42-9c0e-f5a085c6351e@redhat.com>
[not found] ` <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com>
[not found] ` <f0254740-944d-201b-9a66-9db1fe480ca6@redhat.com>
[not found] ` <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com>
[not found] ` <20210715102249.2205-1-hdanton@sina.com>
[not found] ` <20210716020611.2288-1-hdanton@sina.com>
[not found] ` <20210716075539.2376-1-hdanton@sina.com>
[not found] ` <20210716093725.2438-1-hdanton@sina.com>
[not found] ` <a2f3f9ac-dac2-eadc-269e-91652d78ebd3@redhat.com>
2021-07-18 12:42 ` 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() Hillf Danton
2021-07-19 15:38 ` Paolo Bonzini [this message]
2021-07-21 7:04 ` Hillf Danton
2021-07-21 7:25 ` Thomas Gleixner
2021-07-21 10:11 ` Hillf Danton
2021-07-21 10:59 ` Paolo Bonzini
2021-07-22 5:58 ` Hillf Danton
2021-07-23 2:23 ` Hillf Danton
2021-07-23 7:59 ` Paolo Bonzini
2021-07-23 9:48 ` Hillf Danton
2021-07-23 10:56 ` Paolo Bonzini
2021-07-24 4:33 ` Hillf Danton
2021-07-26 11:03 ` Paolo Bonzini
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