Re: [PATCH 2/2] hostmem: detect host backend memory is being used properly

From: Paolo Bonzini <pbonzini@redhat.com>
To: Igor Mammedov <imammedo@redhat.com>,
	Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: ehabkost@redhat.com, kvm@vger.kernel.org, mst@redhat.com,
	gleb@kernel.org, mtosatti@redhat.com, qemu-devel@nongnu.org,
	stefanha@redhat.com, rth@twiddle.net
Subject: Re: [PATCH 2/2] hostmem: detect host backend memory is being used properly
Date: Wed, 13 Jul 2016 12:49:31 +0200	[thread overview]
Message-ID: <e20cd32e-ce44-6c6c-fab1-8f51086e9727@redhat.com> (raw)
In-Reply-To: <20160713093044.356d052d@nial.brq.redhat.com>

On 13/07/2016 09:30, Igor Mammedov wrote:
> I wonder if it could be cleaner to extend QOM API with
> 
> object_get_refcount();
> 
> and then add
> 
> bool hostmem_is_busy()
> {
>     return object_get_refcount() > 1;
> }
> 
> that would work as not used used hostmem would have ref counter == 1
> and when front-end starts to use it, it calls
>   qdev_prop_allow_set_link_before_realize()
> which rises ref counter of backend to 2.
> 
> Also see a comment below.

I disagree---there are many reasons why a refcount can be > 1, and given
that we use RCU the release of the reference can be delayed arbitrarily.

Paolo