On Fri, 2007-06-15 at 19:52 +0400, Oleg Nesterov wrote:
> john stultz wrote:
> >
> > The following additional patch should correct this issue. Although since
> > we weren't actually hitting it, the issue is a bit theoretical, so I've
> > not been able to prove it really fixes anything.
>
> Could you please look at the message below? I sent it privately near a month
> ago, but I think these problems are not fixed yet.
Hmm. Maybe you sent it to others on the cc list, as I can't find it in
my box. But apologies anyway.
> ---------------------------------------------------------------------------
>
> > +__tasklet_action(struct softirq_action *a, struct tasklet_struct *list)
> > +{
> > + int loops = 1000000;
> >
> > while (list) {
> > struct tasklet_struct *t = list;
> >
> > list = list->next;
> > + /*
> > + * Should always succeed - after a tasklist got on the
> > + * list (after getting the SCHED bit set from 0 to 1),
> > + * nothing but the tasklet softirq it got queued to can
> > + * lock it:
> > + */
> > + if (!tasklet_trylock(t)) {
> > + WARN_ON(1);
> > + continue;
> > + }
> > +
> > + t->next = NULL;
> > +
> > + /*
> > + * If we cannot handle the tasklet because it's disabled,
> > + * mark it as pending. tasklet_enable() will later
> > + * re-schedule the tasklet.
> > + */
> > + if (unlikely(atomic_read(&t->count))) {
> > +out_disabled:
> > + /* implicit unlock: */
> > + wmb();
> > + t->state = TASKLET_STATEF_PENDING;
>
> What if tasklet_enable() happens just before this line ?
>
> After the next schedule_tasklet() we have all bits set: SCHED, RUN, PENDING.
> The next invocation of __tasklet_action() clears SCHED, but
> tasklet_tryunlock()
> below can never succeed because of PENDING.
Yep. I've only been focusing on races in schedule/action, as I've been
hunting issues w/ rcu. But I'll agree that the other state changes look
problematic. I know Paul McKenney was looking at some of the other state
changes and was seeing some potential problems as well.
thanks
-john
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