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Date: Mon, 12 May 2014 12:14:25 +0200
From: Paolo Bonzini <pbonzini@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: "Michael S. Tsirkin" <mst@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>

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