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

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Davide Libenzi wrote:
> On Tue, 15 May 2007, Christoph Hellwig wrote:
>
>   
>> 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.
>>     
>
> Right, you could use the file_count() macro :)
> Seriuosly, if this (doing an eventfd_fget+fput) becomes a problem, I can
> have the check done in eventfd_signal() and return a proper error code.
>
>   

IMO doing eventfd_fget() asap is best.  I much prefer refcounted 
pointers to handles in the kernel: it's easier to see what things point 
to, and there is to context needed for dereferencing.

-- 
error compiling committee.c: too many arguments to function


-------------------------------------------------------------------------
This SF.net email is sponsored by DB2 Express
Download DB2 Express C - the FREE version of DB2 express and take
control of your XML. No limits. Just data. Click to get it now.
http://sourceforge.net/powerbar/db2/