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

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Message-ID: <4A3A8FBC.10905@novell.com>
Date:	Thu, 18 Jun 2009 15:04:28 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Davide Libenzi <davidel@...ilserver.org>
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

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.

Later, after we get this thing all built, I will re-run the numbers and
post some results so that Davide may have better proof that the context
switch overhead isn't just lost in the noise.  I think that is all he is
asking for anyway, which is understandable.

Regards,
-Greg

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