Re: [PATCH 5/9] KVM: Adds ability to signal userspace using a file-descriptor

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Tue, May 15, 2007 at 12:18:17PM -0400, Gregory Haskins wrote:
> >>> On Tue, May 15, 2007 at 11:40 AM, in message
> <Pine.LNX.4.64.0705150832130.30085-GPJ85BhbkB8RepQJljzAVbITYcZ0+W3JAL8bYrjMMd8@public.gmane.org>, Davide Libenzi
> <davidel-AhlLAIvw+VEjIGhXcJzhZg@public.gmane.org> wrote: 
> >
> > I don't know how critical is the path where you will be doing check. The 
> > eventfd_fget() is pretty fast, so if you're not looking at a performance 
> > critical path, I'd suggest that. Otherwise you can do an early 
> > eventfd_get, and keep the file*. If you have no the ways to know if the 
> > userspace disconnected, an atomic_read(&file- >f_count)==1 will tell you 
> > that you're the only owner of the file* (that is, userspace closed the 
> > eventfd descriptor). I'd give preference to the former option though.
> 
> Thanks for the insight, Davide.  It sounds like we can probably stay with the way I have it for now, and keep the file->f_count idea in our back pocket should a performance problem arise.

accessing file->f_count is not allowed for drivers.  It took us quite a
bit of effort to clean up all users a while ago, and it turned out most
of them were rather buggy.


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