On Mon, Jan 26, 2015 at 10:08:04AM +0800, Lai Jiangshan wrote:
> On 01/25/2015 05:18 AM, Paul E. McKenney wrote:
>
> >
> > Good point! In my scenario, CPU 0 would not yet have switched away from
> > Task A. Hmmm... Yet Sasha really does see this failure. Will give it
> > some more thought.
> >
> > Any ideas?
>
> I don't known which commit was merged from the rcu-git-tree in Sasha's test
> I try to review it.
If I had to guess, it would be 1d082fd06188 (Remove local_irq_disable()
in rcu_preempt_note_context_switch()), though his finding this might be
more directly related to increases in trinity's levels of stress.
> We can fallback to git-bitsect if the reviews fails.
One (very unlikely) possibility is that Sasha's compiler is ignoring the
barrier() in rcu_preempt_qs().
Thanx, Paul
> Thanks,
> Lai
>
> >
> > Thanx, Paul
> >
> >> Thanks,
> >> Lai
> >>
> >>>
> >>> 6. Once in rcu_read_unlock_special(), the fact that
> >>> current->rcu_read_unlock_special.b.need_qs is true becomes
> >>> apparent, so rcu_read_unlock_special() invokes rcu_preempt_qs().
> >>> Recursively, given that we interrupted out of that same
> >>> function in the preceding step.
> >>>
> >>> 7. Because rcu_preempt_data.passed_quiesce is now true,
> >>> rcu_preempt_qs() does nothing, and simply returns.
> >>>
> >>> 8. Upon return to rcu_read_unlock_special(), it is noted that
> >>> current->rcu_read_unlock_special is still nonzero (because
> >>> the interrupted rcu_preempt_qs() had not yet gotten around
> >>> to clearing current->rcu_read_unlock_special.b.need_qs).
> >>>
> >>> 9. Execution proceeds to the WARN_ON_ONCE(), which notes that
> >>> we are in an interrupt handler and thus duly splats.
> >>>
> >>> The solution, as noted above, is to make rcu_read_unlock_special()
> >>> clear out current->rcu_read_unlock_special.b.need_qs after calling
> >>> rcu_preempt_qs(). The interrupted rcu_preempt_qs() will clear it again,
> >>> but this is harmless. The worst that happens is that we clobber another
> >>> attempt to set this field, but this is not a problem because we just
> >>> got done reporting a quiescent state.
> >>>
> >>> Reported-by: Sasha Levin <[email protected]>
> >>> Signed-off-by: Paul E. McKenney <[email protected]>
> >>>
> >>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> >>> index 8669de884445..ec99dc16aa38 100644
> >>> --- a/kernel/rcu/tree_plugin.h
> >>> +++ b/kernel/rcu/tree_plugin.h
> >>> @@ -322,6 +322,7 @@ void rcu_read_unlock_special(struct task_struct *t)
> >>> special = t->rcu_read_unlock_special;
> >>> if (special.b.need_qs) {
> >>> rcu_preempt_qs();
> >>> + t->rcu_read_unlock_special.need_qs = false;
> >>> if (!t->rcu_read_unlock_special.s) {
> >>> local_irq_restore(flags);
> >>> return;
> >>>
> >>> .
> >>>
> >>
> >
> > .
> >
>
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