linux-kernel - Re: [PATCH v5 1/4] kvm: Extend irqfd to support level interrupts

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Message-ID: <20120717222850.GK1868@redhat.com>
Date:	Wed, 18 Jul 2012 01:28:51 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	avi@...hat.com, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, jan.kiszka@...mens.com
Subject: Re: [PATCH v5 1/4] kvm: Extend irqfd to support level interrupts

On Tue, Jul 17, 2012 at 04:16:04PM -0600, Alex Williamson wrote:
> On Wed, 2012-07-18 at 01:00 +0300, Michael S. Tsirkin wrote:
> > On Tue, Jul 17, 2012 at 03:57:41PM -0600, Alex Williamson wrote:
> > > On Wed, 2012-07-18 at 00:26 +0300, Michael S. Tsirkin wrote:
> > > > On Mon, Jul 16, 2012 at 02:33:47PM -0600, Alex Williamson wrote:
> > > > > @@ -96,6 +183,9 @@ irqfd_shutdown(struct work_struct *work)
> > > > >  	 * It is now safe to release the object's resources
> > > > >  	 */
> > > > >  	eventfd_ctx_put(irqfd->eventfd);
> > > > > +
> > > > > +	_irq_source_put(irqfd->source);
> > > > > +
> > > > >  	kfree(irqfd);
> > > > >  }
> > > > >  
> > > > 
> > > > Hang on, this is a bug I think. This is done asynchronously.  So this
> > > > means that I can assign MAX number of irqfds, then close one, and now
> > >                          ^^^^^^^^^^^^^^^^^^^^ What is this?
> > > Do you mean max irq source ids?
> > 
> > Yes, this is what I meant. Sorry about being unclear.
> > 
> > > > assign will fail because deassign did not get freed.
> > > > 
> > > > Maybe we can solve this by flushing wq before assign?
> > > > Looks a bit fragile but may be enough - need to document well.
> 
> Why is this fragile?  We could even make it part of a retry so we don't
> call it unless we need to.
> 

It just ties in assign and deassign. Maybe it's ok - but pls
add a comment explaining the whole design.

-- 
MST
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