Subject: Re: [PATCH] sched: Optimize return value of qdisc_restart

From: jamal <[email-masked]>
Date: Thu, 10 May 2007 08:21:01 -0400

On Thu, 2007-10-05 at 04:55 -0700, David Miller wrote:
> From: Herbert Xu <[email-masked]>
> Date: Thu, 10 May 2007 21:50:39 +1000
> 

> T_SCHED]: Rationalise return value of qdisc_restart
> > 
> > The current return value scheme and associated comment was invented
> > back in the 20th century when we still had that tbusy flag.  Things
> > have changed quite a bit since then (even Tony Blair is moving on
> > now, not to mention the new French president).
> > 
> > All we need to indicate now is whether the caller should continue
> > processing the queue.  Therefore it's sufficient if we return 0 if
> > we want to stop and non-zero otherwise.
> > 
> > This is based on a patch by Krishna Kumar.
> > 
> > Signed-off-by: <[email-masked]>
> 
> Fair enough, patch applied :-)

Ok, see if this makes sense:

CPU0                 CPU1 (holding qdisc running)
  .                    |
  .                    |
  .                    + grab qlock
  .                    |
  .                    | deq pkt
  .                    + release qlock
  .                    + grab txlock
  .                    |  send pkt
  .                    + release txlock
  .                    + grab qlock
   has pktX            | (NEW: qlen = 0); return 0 instead of -1
   waiting for qlock   + release qlock
  + grab qlock         |
  |                    + find that return code is 0
  | enq pkt X          + release qdisc running
  |                    |
  + release qlock      | ==> outta here

pkt X is stuck unless some event happens such as a new pkt arrival. 
In other words it sits there for an indeterminate period.

cheers,
jamal