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Message-ID: <4ACB7794.5040308@gmail.com>
Date: Tue, 06 Oct 2009 13:00:04 -0400
From: Gregory Haskins <gregory.haskins@gmail.com>
User-Agent: Thunderbird 2.0.0.23 (Macintosh/20090812)
MIME-Version: 1.0
To: Avi Kivity <avi@redhat.com>
CC: Gregory Haskins <ghaskins@novell.com>, kvm@vger.kernel.org,
       linux-kernel@vger.kernel.org,
       "alacrityvm-devel@lists.sourceforge.net" 
	<alacrityvm-devel@lists.sourceforge.net>,
       David Howells <dhowells@redhat.com>
Subject: Re: [PATCH v2 2/4] KVM: introduce "xinterface" API for external	interaction
 with guests
References: <20091002201159.4014.33268.stgit@dev.haskins.net> <20091002201927.4014.29432.stgit@dev.haskins.net> <4AC8780D.1060800@redhat.com> <4ACA87D7.1080206@gmail.com> <4ACB0F3C.1000705@redhat.com> <4ACB46AD.8010405@gmail.com> <4ACB528D.6030408@gmail.com> <4ACB6F0E.4000407@redhat.com>
In-Reply-To: <4ACB6F0E.4000407@redhat.com>
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Avi Kivity wrote:
> On 10/06/2009 04:22 PM, Gregory Haskins wrote:
>>>>>>> +
>>>>>>> +static inline void
>>>>>>> +_kvm_xinterface_release(struct kref *kref)
>>>>>>> +{
>>>>>>> +    struct kvm_xinterface *intf;
>>>>>>> +    struct module *owner;
>>>>>>> +
>>>>>>> +    intf =3D container_of(kref, struct kvm_xinterface, kref);
>>>>>>> +
>>>>>>> +    owner =3D intf->owner;
>>>>>>> +    rmb();
>>>>>>>
>>>>>>>
>>>>>>>             =20
>>>>>> Why rmb?
>>>>>>
>>>>>>           =20
>>>>> the intf->ops->release() line may invalidate the intf pointer, so w=
e
>>>>> want to ensure that the read completes before the release() is call=
ed.
>>>>>
>>>>> TBH: I'm not 100% its needed, but I was being conservative.
>>>>>
>>>>>         =20
>>>> rmb()s are only needed if an external agent can issue writes, otherw=
ise
>>>> you'd need one after every statement.
>>>>       =20
>>> I was following lessons learned here:
>>>
>>> http://lkml.org/lkml/2009/7/7/175
>>>
>>> Perhaps mb() or barrier() are more appropriate than rmb()?  I'm CC'in=
g
>>> David Howells in case he has more insight.
>>>     =20
>> BTW: In case it is not clear, the rationale as I understand it is we
>> worry about the case where one cpu reorders the read to be after the
>> ->release(), and another cpu might grab the memory that was kfree()'d
>> within the ->release() and scribble something else on it before the re=
ad
>> completes.
>>
>> I know rmb() typically needs to be paired with wmb() to be correct, so=

>> you are probably right to say that the rmb() itself is not appropriate=
=2E
>>   This problem in general makes my head hurt, which is why I said I am=

>> not 100% sure of what is required.  As David mentions, perhaps
>> "smp_mb()" is more appropriate for this application.  I also speculate=

>> barrier() may be all that we need.
>>   =20
>=20
> barrier() is the operation for a compiler barrier.  But it's unneeded
> here - unless the compiler can prove that ->release(intf) will not
> modify intf->owner it is not allowed to move the access afterwards.  An=

> indirect function call is generally a barrier() since the compiler can'=
t
> assume memory has not been modified.
>=20

You're logic seems reasonable to me.  I will defer to David, since he
brought up the issue with the similar logic originally.

Kind Regards,
-Greg


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