linux-kernel - Re: [PATCH v2 2/4] KVM: introduce "xinterface" API for external interaction with guests

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Message-ID: <4ACB6F0E.4000407@redhat.com>
Date:	Tue, 06 Oct 2009 18:23:42 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Gregory Haskins <gregory.haskins@...il.com>
CC:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"alacrityvm-devel@...ts.sourceforge.net" 
	<alacrityvm-devel@...ts.sourceforge.net>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH v2 2/4] KVM: introduce "xinterface" API for external	interaction
 with guests

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 = container_of(kref, struct kvm_xinterface, kref);
>>>>>> +
>>>>>> +    owner = intf->owner;
>>>>>> +    rmb();
>>>>>>
>>>>>>
>>>>>>              
>>>>> Why rmb?
>>>>>
>>>>>            
>>>> the intf->ops->release() line may invalidate the intf pointer, so we
>>>> want to ensure that the read completes before the release() is called.
>>>>
>>>> TBH: I'm not 100% its needed, but I was being conservative.
>>>>
>>>>          
>>> rmb()s are only needed if an external agent can issue writes, otherwise
>>> you'd need one after every statement.
>>>        
>> 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'ing
>> David Howells in case he has more insight.
>>      
> 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 read
> 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.
>   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.
>    

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.

-- 
error compiling committee.c: too many arguments to function

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