On Tue, Aug 19, 2008 at 07:35:58AM +0000, Jarek Poplawski wrote:
quoted text
>
> > If we're removing a non-root qdisc, then we will first grab the
> > root qdisc's lock, kill the child, and release the root lock.  By
> > convention, any user of a child qdisc must have acquired the root
> > qdisc's lock because the child is only reachable through the root.
> > Therefore once we have released the root qdisc lock after killing
> > the child, we're guaranteed that no further references to that
> > child can be made.
> 
> By convention, there was always this comment that destroy is under
> sch_tree_lock(), so it was legal to depend on this. I'm not afraid
> of somebody using such an under destroy qdisc - it's about a code
> inside this qdisc could refer to not destroyed part.

No no no, it's not about qdisc_destroy at all.  If you're relying
on the lock around qdisc_destroy, then you're already too late.
The qdisc should have been removed before we get to qdisc_destroy.

It's the act of removal that's protected by the root lock, and
still is.  For example, in htb_graft we do sch_tree_lock before
killing any children, this is the lock that I was referring to.

quoted text
> > If for whatever reason the code does not reflect the reasoning
> > above, please feel free to fix the code :)
> 
> Sure, I'll try to look for such problems.

Of course if you find any classful qdisc that does not hold the
tree lock when killing children, then please send patches.

Thanks,
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