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Message-ID: <45e643bce58e0f7c9646bb6c548c4e9f026f1fa8.camel@linux.ibm.com>
Subject: Re: [PATCH] KVM: Add separate helper for putting borrowed reference
 to kvm
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        kvm-ppc@vger.kernel.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Date:   Thu, 28 Nov 2019 13:29:00 -0300
In-Reply-To: <20191128010001.GJ22227@linux.intel.com>
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On Wed, 2019-11-27 at 17:00 -0800, Sean Christopherson wrote:
> > Sorry, I missed some information on above example.=20
> > Suppose on that example that the reorder changes take place so that
> > kvm_put_kvm{,_no_destroy}() always happens after the last usage of kvm
> > (in the same syscall, let's say).
>=20
> That can't happen, because the ioctl() holds a reference to KVM via its
> file descriptor for /dev/kvm, and ioctl() in turn prevents the fd from
> being closed.
>=20
> > Before T1 and T2, refcount =3D 1;
>=20
> This is what's impossible.  T1 must have an existing reference to get
> into the ioctl(), and that reference cannot be dropped until the ioctl()
> completes (and by completes I mean returns to userspace). Assuming no
> other bugs, i.e. T2 has its own reference, then refcount >=3D 2.
>=20

Thanks for explaining, I think I get it now.

Best regards,
Leonardo Bras

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