Re: upcalls from kernel code to user space daemons

[Chris Friesen <cfriesen@nortelnetworks.com>]

Bernd Petrovitsch wrote:
> On Mon, 2004-06-14 at 23:57, Chris Friesen wrote:
>  > Oliver Neukum wrote:
>  >
>  > >  > userspace daemon loops on ioctl()
>  > >  > kernel portion of ioctl call goes to sleep until something to do
>  > >  > when needed, fill in data and return to userspace
>  > >  > userspace does stuff, then passes data back down via ioctl()
>  > >  > ioctl() puts userspace back to sleep and continues on with other 
> work
>  > >
>  > > You could just as well implement an ordinary read()
>  >
>  > Not quite.  The userspace is passing data down as well.  I don't know 
> how you'd
>  > do that with read().
> 
> For this you use write().

And you eat another syscall per userspace call.  For the ioctl() case, you only 
need to issue a single call to ioctl().  You pass the result of the previous 
request down, and then when it returns you get the data for the next request.

Although I have to admit it's not pretty, and the performance improvements may 
not be worth the obfuscation of the code.

Chris