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

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Message-ID: <20090618064908.GB11155@redhat.com>
Date:	Thu, 18 Jun 2009 09:49:10 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Gregory Haskins <ghaskins@...ell.com>, 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

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!

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.
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.

-- 
MST
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