linux-kernel - Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

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Date:	Thu, 18 Jun 2009 15:03:19 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com, paulmck@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier
 interface

On Thu, 18 Jun 2009, Gregory Haskins wrote:

> Davide Libenzi wrote:
> > On Thu, 18 Jun 2009, Gregory Haskins wrote:
> >
> >   
> >> Actually there is only one (the tx-thread) aside from the eventfd
> >> imposed workqueue one.  Incidentally, I would love to get rid of the
> >> other thread too, so I am not just picking on eventfd ;).  The other is
> >> a lot harder since it has to update the virtio-ring and may need to page
> >> in guest memory to do so.
> >>     
> >
> > No, there is the interface rx softirq too, that makes two.
> 
> Actually, I believe you are mistaken.  It normally executes the softirq
> in interrupt context, not a thread.
> 
> But I digress.  Lets just shelve the SRCU conversation for another day. 
> It was my bad for introducing it now prematurely to solve a mostly
> unrelated problem: the module-reference thing.  I didn't realize the
> SRCU change would be so controversial, and I didn't think to split
> things apart as I have done today.
> 
> But the fact is: I do not see any way to actually use your referenceless
> POLLHUP release code in a race free way without doing something like I
> propose in 3/4, 4/4.   Lets keep the discussion focused on that for now,
> if we could.

OK, since I got literally swamped by the amount of talks and patches over 
this theoretically simple topic, would you mind  1) posting the global 
patch over eventfd  2) describe exactly what races are you talking about 
3) explain why this should be any concern of eventfd at all?



- Davide


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