Re: [PATCHv2 1/3] eventfd: allow atomic read and waitqueue remove

[Avi Kivity]

On 01/21/2010 08:02 PM, Michael S. Tsirkin wrote:
> On Thu, Jan 21, 2010 at 07:57:22PM +0200, Avi Kivity wrote:
>    
> > On 01/21/2010 07:56 PM, Avi Kivity wrote:
> >      
> > > On 01/21/2010 07:45 PM, Michael S. Tsirkin wrote:
> > >        
> > > >          
> > > > > But you're in process context.  An eventfd never blocks.
> > > > >            
> > > > Yes it blocks if counter is 0. And we don't know
> > > > it's not 0 unless we read :) catch-22.
> > > >          
> > > Ah yes, I forgot.
> > >
> > >        
> > Well, you can poll it and then read it... this introduces a new race (if
> > userspace does a read in parallel) but it's limited to kvm and buggy
> > userspace.
> >      
> I would rather not require that userspace never reads this fd.
> You are right that it does not now, but adding this as requirement
> looks like exporting an implementation bug to userspace.
>    

Well, I don't like risking 2.6.32 non-kvm users for a bug that doesn't 
happen in practice now.

After it gets some time in 2.6.33, we can backport it to 2.6.32 (since 
that will be maintained long term).

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

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