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:	Mon, 15 Jun 2009 15:54:39 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	davidel@...ilserver.org, paulmck@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead
	of an explicit ioctl

On Mon, Jun 15, 2009 at 08:08:18AM -0400, Gregory Haskins wrote:
> >> @@ -123,6 +124,7 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int
> >> sync, void
> >>  *key)
> >>  
> >>                 cleanup_srcu_struct(&irqfd->srcu);
> >>                 kfree(irqfd);
> >> +               module_put(THIS_MODULE);
> >>         }
> >>  
> >>         return 0;
> >>     
> >
> > module_put(THIS_MODULE) is always a bug unless you know that someone has
> > a reference to the current module: the module could go away between this
> > call and returning from function.
> >   
> 
> 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?


-- 
MST
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