On Sat, 2005-10-29 at 10:51 -0400, Alan Stern wrote:
> On Fri, 28 Oct 2005, Chandra Seetharaman wrote:
>
> > On Fri, 2005-10-28 at 10:23 -0400, Alan Stern wrote:
> > > On Thu, 27 Oct 2005, Chandra Seetharaman wrote:
> > >
> > > > So, requirements to fix the bug are:
> > > > - no sleeping in register/unregister(if we want to keep the
> > > > current way of use. We can change it and make the relevant
> > > > changes in the kernel code, if it is agreeable)
> > >
> > > I think we will have to make these changes. In principal it shouldn't be
> > > hard to add a simple "enabled" flag to each callout which currently is
> > > registered/unregistered atomically or while running. We could even put
> > > such a flag into the notifier_block structure and add routines to set or
> > > clear it, using appropriate barriers.
> >
> > I do not understand the purpose of enabled flag. Can you clarify
>
> Something like this:
>
> struct notifier_block {
> int (*notifier_call)(struct notifier_block *self, unsigned long,
> void *);
> struct list_head node;
> int priority;
> int enabled;
> };
>
> int notifier_call_chain(struct notifier_head *nh, unsigned long val,
> void *v)
> {
> int ret = 0;
> notifier_block *b;
>
> if (list_empty(&nh->chain)) /* Optimize for common case */
> return ret;
>
> smp_rmb();
> list_for_each_entry(b, &nh->chain, node) {
> if (b->enabled) {
> ret = b->notifier_call(b, val, v);
> if (ret & NOTIFY_STOP_MASK)
> break;
> }
> }
>
> return ret;
> }
>
> #define notifier_block_enable(b) set_wmb((b)->enabled, 1)
> #define notifier_block_disable(b) set_wmb((b)->enabled, 0)
>
I am not getting the complete picture. So, in unregister we would just
disable and never delete the notifier_block ? Or
notifier_block_enable/disable will be used by external entities
directly ?
>
> It occurred to me that there _is_ a way to do unregister for atomic chains
> without blocking. Add to struct notifier_head
>
> atomic_t num_callers;
>
> Then in notifier_call_chain, do atomic_inc(&nh->num_callers) at the start
> and atomic_dec(&nh->num_callers) at the end. Finally, make unregister do
> this:
>
> int notifier_chain_unregister(struct notifier_head *nh,
> struct notifier_block *n)
> {
> if (nh->type == ATOMIC_NOTIFIER) {
> spin_lock(nh->lock);
> list_del(&n->node);
> smp_mb();
> while (atomic_read(&nh->num_callers) > 0)
> cpu_relax();
> spin_unlock(nh->lock);
> } else {
> ...
> }
> return 0;
> }
But, how is the list protected in call_chain (will you be holding the
lock in call_chain() while incrementing the atomic variable).
>
> I don't mean to suggest that this is better than using RCU, and with
> notifier_block_disable it probably isn't needed. However it is worth
> thinking about.
>
> Alan Stern
>
>
--
----------------------------------------------------------------------
Chandra Seetharaman | Be careful what you choose....
- [email protected] | .......you may get it.
----------------------------------------------------------------------
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