Messages in this thread | | | Date | Tue, 27 Jan 2015 14:03:29 -0800 | From | "Paul E. McKenney" <> | Subject | Re: rcu, sched: WARNING: CPU: 30 PID: 23771 at kernel/rcu/tree_plugin.h:337 rcu_read_unlock_special+0x369/0x550() |
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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 <sasha.levin@oracle.com> > >>> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > >>> > >>> 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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