linux-kernel - Re: [PATCHv2] kvm: remove in_range and switch to rwsem for iobus

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Date:	Mon, 29 Jun 2009 13:21:23 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Gregory Haskins <ghaskins@...ell.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	paulmck@...ux.vnet.ibm.com, markmc@...hat.com
Subject: Re: [PATCHv2] kvm: remove in_range and switch to rwsem for iobus

On 06/29/2009 01:06 PM, Michael S. Tsirkin wrote:
>>> 2. if I do it before in_range removal it's a lot of churn:
>>> one of the reasons for code reorg is so that there are less
>>> places to change locking.
>>>
>>>
>>>
>>>        
>> I don't think you really need to change anything.  slots_lock is already
>> taken (except where you modify the list).
>>      
>
> Are you sure about this? I don't understand the code well enough, so
> this reuse of an apparently unrelated lock just makes me nervious.  For
> example what about emulate_instruction? It is sometimes called from
> svm/vmx without slot lock ...
>    

vcpu context always has slots lock taken IIRC, except when in guest mode.

It's not an unrelated lock; slots lock locks memory hotplug, we extend 
it to lock mmio_bus and io_bus hotplug.

I'd really like to avoid a proliferation of locks.

>> How about this:
>>
>> 1. add slots_lock for write when modifying the list
>> 2. change the api
>> 3. drop kvm->lock
>>
>> ?
>>      
>
> Looks like I will just have to bite the bullet and switch to RCU.
>
>    

You still need a lock to prevent concurrent modifications to mmio_bus 
(but can use kvm->lock for this).


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

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