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

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Message-ID: <4A488F2C.2060400@redhat.com>
Date:	Mon, 29 Jun 2009 12:53:48 +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 12:41 PM, Michael S. Tsirkin wrote:
> On Mon, Jun 29, 2009 at 11:37:00AM +0300, Avi Kivity wrote:
>    
>> On 06/28/2009 10:34 PM, Michael S. Tsirkin wrote:
>>      
>>> This changes bus accesses to use high-level kvm_io_bus_read/kvm_io_bus_write
>>> functions, which utilize read/write semaphore intead of mutex.  in_range now
>>> becomes unused so it is removed from device ops in favor of read/write
>>> callbacks performing range checks internally.
>>>
>>> This allows aliasing (mostly for in-kernel virtio), as well as better error
>>> handling by making it possible to pass errors up to userspace. And it's enough
>>> to look at the diffstat to see that it's a better API anyway.
>>>
>>> While we are at it, document locking rules for kvm_io_device_ops.
>>>
>>> Note: since the use of the new bus_lock is localized to a small number of
>>> places, it will be easy to switch to srcu in the future if we so desire.
>>>
>>>        
>> Looks good. But please split into a locking change patch and an API
>> change patch (in whatever order makes more sense).
>>      
>
> This is harder than it seems. Is this really important?
>
> The locking change itself is about 6 lines, but
> 1. if I do it after in_range removal I get deadlocks
> as after marcelo's change kvm->lock is taken internally by writers.
>    

slots_lock is an outer lock to kvm->lock.

> 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).

How about this:

1. add slots_lock for write when modifying the list
2. change the api
3. drop kvm->lock

?

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

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