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To:     Hillf Danton <hdanton@sina.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Message-ID: <724e7951-59c8-8b2b-37b8-6b0bf696ab04@redhat.com>
Date:   Mon, 19 Jul 2021 17:38:45 +0200
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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.
>