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Date: Mon, 12 May 2014 13:18:44 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] vhost: Can we change synchronize_rcu to
 call_rcu in vhost_set_memory() in vhost kernel module?
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Huangweidong (C)" <weidong.huang@huawei.com>, "gleb@redhat.com" <gleb@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Gonglei (Arei)" <arei.gonglei@huawei.com>, "avi.kivity@gmail.com" <avi.kivity@gmail.com>, "Herongguang (Stephen)" <herongguang.he@huawei.com>

On Mon, May 12, 2014 at 12:14:25PM +0200, Paolo Bonzini wrote:
> Il 12/05/2014 12:08, Michael S. Tsirkin ha scritto:
> >On Mon, May 12, 2014 at 11:57:32AM +0200, Paolo Bonzini wrote:
> >>Perhaps we can check for cases where only the address is changing,
> >>and poke at an existing struct kvm_kernel_irq_routing_entry without
> >>doing any RCU synchronization?
> >
> >I suspect interrupts can get lost then: e.g. if address didn't match any
> >cpus, now it matches some. No?
> 
> Can you explain the problem more verbosely? :)
> 
> Multiple writers would still be protected by the mutex, so you
> cannot have an "in-place update" writer racing with a "copy the
> array" writer.
> 
> Paolo

I am not sure really.
I'm worried about reader vs writer.
If reader sees a stale msi value msi will be sent to a wrong
address.