On 06/15, john stultz wrote:
>
> On Fri, 2007-06-15 at 19:52 +0400, Oleg Nesterov wrote:
> >
> > 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.
checking my mbox... Oops, you are right, sorry!
> > > + 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.
OK, thanks. But doesn't this mean your 2-nd patch is questionable?
> + } else {
> + /* This is subtle. If we hit the corner case above
> + * It is possible that we get preempted right here,
> + * and another task has successfully called
> + * tasklet_schedule(), then this function, and
> + * failed on the trylock. Thus we must be sure
> + * before releasing the tasklet lock, that the
> + * SCHED_BIT is clear. Otherwise the tasklet
> + * may get its SCHED_BIT set, but not added to the
> + * list
> + */
> + if (!tasklet_tryunlock(t))
> + goto again;
Again, tasklet_tryunlock() can fail because _PENDING was set by __tasklet_action().
In that case __tasklet_common_schedule() goes to the endless loop, no?
Oleg.
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