linux-kernel - Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead of an explicit ioctl

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Date:	Thu, 18 Jun 2009 08:00:39 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, paulmck@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org
Subject: Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead
 of an explicit ioctl

Michael S. Tsirkin wrote:
> On Thu, Jun 18, 2009 at 02:46:30PM +0930, Rusty Russell wrote:
>   
>> On Mon, 15 Jun 2009 10:24:39 pm Michael S. Tsirkin wrote:
>>     
>>> On Mon, Jun 15, 2009 at 08:08:18AM -0400, Gregory Haskins wrote:
>>>       
>>>> Hmm.  I understand what you are saying conceptually (i.e. the .text
>>>> could get yanked before we hit the next line of code, in this case the
>>>> "return 0").  However, holding a reference when you _know_ someone else
>>>> holds a reference to me says that one of the references is redundant.
>>>> In addition, there is certainly plenty of precedence for
>>>> module_put(THIS_MODULE) all throughout the kernel (including
>>>> module_put_and_exit()).  Are those broken as well?
>>>>         
>>> Maybe not, but I don't know why. It works fine as long as you don't
>>> unload any modules though :) Rusty, could you enlighten us please?
>>>       
>> Yep, they're almost all broken.  A few have comments indicating that someone 
>> else is holding a reference (eg. loopback).
>>
>> But at some point you give up playing whack-a-mole for random drivers.
>>
>> module_put_and_exit() does *not* have this problem, BTW.
>>
>> Rusty.
>>     
>
> I see that, the .text for module_put_and_exit is never modular itself.
> Thanks, Rusty!
>   

Ah!  That is the trick I wasn't understanding.
> BTW, Gregory, this can be used to fix the race in the design: create a
> thread and let it drop the module reference with module_put_and_exit.
>   

I had thought of doing something like this initially too, but I think
its racy as well.  Ultimately, you need to make sure the eventfd
callback is completely out before its safe to run, and deferring to a
thread would not change this race.  The only sane way I can see to do
that is to have the caller infrastructure annotate the event somehow
(either directly with a module_put(), or indirectly with some kind of
state transition that can be tracked with something like
synchronize_sched().
> Which will work, but I guess at this point we should ask ourselves
> whether all the hearburn with srcu, threads and module references is
> better than just asking the user to call and ioctl.
>   

I am starting to agree with you, here. :)

Note one thing: the SRCU stuff is mostly orthogonal from the rest of the
conversation re: the module_put() races.  I only tied it into the
current thread because the eventfd_notifier_register() thread gave me a
convenient way to hook some other context to do the module_put().  In
the long term, the srcu changes are for the can_sleep() stuff.  So on
that note, lets see if I can convince Davide that the srcu stuff is not
so evil before we revert the POLLHUP patches, since the module_put() fix
is trivial once that is in place.

Thanks Michael (and Rusty),
-Greg



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