On Tue, Aug 19, 2008 at 05:46:06PM +1000, Herbert Xu wrote:

quoted text
> On Tue, Aug 19, 2008 at 07:35:58AM +0000, Jarek Poplawski wrote:

> >

> > > 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.

I'm not sure I can understand you: could you look at htb_destroy()

instead and think of this as a child qdisc of prio or another htb?

Having a top level "queue" lock guarantees there is no activity at

the whole tree at the moment.
Thanks,

Jarek P.

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