From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc
Date: Thu, 5 Dec 2013 22:22:44 -0200
Message-ID: <20131206002244.GA29960@amt.cnet>
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Cc: Gleb Natapov <gleb@redhat.com>, avi.kivity@gmail.com,
	"pbonzini@redhat.com Bonzini" <pbonzini@redhat.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Eric Dumazet <dada1@cosmosbay.com>,
	Peter Zijlstra <peterz@infradead.org>
To: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
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On Thu, Dec 05, 2013 at 11:30:27PM +0800, Xiao Guangrong wrote:
> In some cases, the lockless walker will do endless-walking on desc an=
d
> without rewalk, consider this case:
>=20
> there are two descs: desc1 and desc2 who is pointed by desc1->next=EF=
=BC=9A
> desc1->next =3D desc2.
>=20
> CPU 0                                                                =
            CPU 1
>=20
> lockless walk on desc1
>                                                                  dele=
ting desc1 from the rmap
> lockless walk on desc2 (desc1->next)
>                                                                 delet=
e desc2 from the rmap
>                                                                 add d=
esc1
>                                                                 add d=
esc2, then desc2->next =3D desc1
>=20
> lockless walk on desc1
>                                                                delete=
 desc2
>                                                                delete=
 desc1
>                                                                add de=
sc2
>                                                                add de=
sc1; the desc1->next =3D desc2
> lockless walk on desc2
>=20
> =E2=80=A6=E2=80=A6
>=20
> Then, the walker is endlessly walking on desc1 and desc2 without any =
rewalk.

The counter can be local to the walker. If its more than maximum rmap
size, break.

(but still, please consider carefully whether lockless list walking is
really necessary or can be avoided).