Re: [syzbot] WARNING in exit_tasks_rcu_finish

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Jun 13, 2022 at 01:55:31PM +0000, Zhang, Qiang1 wrote:
> > syzbot <syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com> writes:
> > 
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    6d0c80680317 Add linux-next specific files for 20220610
> > > git tree:       linux-next
> > > console output: 
> > > https://syzkaller.appspot.com/x/log.txt?x=13b52c2ff00000
> > > kernel config:  
> > > https://syzkaller.appspot.com/x/.config?x=a30d6e3e814e5931
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=9bb26e7c5e8e4fa7e641
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > 
> > I don't understand what is going on in linux-next kernel/rcu/tasks.h 
> > looks different than in Linus's tree.  Paul does that mean you have 
> > some staged rcu changes?
> 
> >Less than 100 RCU-related patches in -rcu, so not all that bad.  ;-)
> >
> >But yes, this could possibly be an issue in one of those patches.
> 
> > Eric
> > 
> > 
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com
> > >
> > > ------------[ cut here ]------------
> > > WARNING: CPU: 1 PID: 28639 at kernel/rcu/tasks.h:1664 
> > > exit_tasks_rcu_finish_trace kernel/rcu/tasks.h:1664 [inline]
> > > WARNING: CPU: 1 PID: 28639 at kernel/rcu/tasks.h:1664 
> > > exit_tasks_rcu_finish+0x122/0x1b0 kernel/rcu/tasks.h:1006
> 
> >The usual way for this warning to trigger is for these a task to exit while in an RCU Tasks Trace read-side critical section:
> >
> >	rcu_read_lock_trace();
> >	do_something_that_causes_task_exit();
> >
> 
> Hi Paul, wether the following scenarios be considered
> 
> rcu_read_unlock_trace_special
>    ->if (trs.b.blocked)
>        ->raw_spin_lock_irqsave_rcu_node
>        ->list_del_init(&t->trc_blkd_node)
>        ->WRITE_ONCE(t->trc_reader_special.b.blocked, false)
>        ->raw_spin_unlock_irqrestore_rcu_node
>                      ->Inerrrupt
>                             ->schedule
>                                 ->rcu_note_context_switch
>                                     ->rcu_tasks_trace_qs
>                                          If (___rttq_nesting && !READ_ONCE((t)->trc_reader_special.b.blocked) 
>                                                         /*___rttq_nesting  ==1    &&  (t)->trc_reader_special.b.blocked =false*/
> 				rcu_tasks_trace_qs_blkd(t)     
>    ->WRITE_ONCE(t->trc_reader_nesting, 0)
>     .......
>    -> exit_tasks_rcu_finish
> 
> Whether the following patch can fix it, or what am I missing?
> Any thoughts?
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index f1209ce621c5..c607e4c914d3 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -1247,6 +1247,7 @@ void rcu_read_unlock_trace_special(struct task_struct *t)
>         struct rcu_tasks_percpu *rtpcp;
>         union rcu_special trs;
> 
> +       WRITE_ONCE(t->trc_reader_nesting, 0);
>         // Open-coded full-word version of rcu_ld_need_qs().
>         smp_mb(); // Enforce full grace-period ordering.
>         trs = smp_load_acquire(&t->trc_reader_special);
> @@ -1267,7 +1268,6 @@ void rcu_read_unlock_trace_special(struct task_struct *t)
>                 WRITE_ONCE(t->trc_reader_special.b.blocked, false);
>                 raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
>         }
> -       WRITE_ONCE(t->trc_reader_nesting, 0);
>  }
>  EXPORT_SYMBOL_GPL(rcu_read_unlock_trace_special);

Thank you for looking into this!

You do have what I believe to be the correct failure scenario, but the
above fix would break nested RCU Tasks Trace read-side critical sections.

But would you be willing to try out the patch shown below?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 08059d8d4f5a7..937a58b3266bf 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -184,7 +184,7 @@ void rcu_tasks_trace_qs_blkd(struct task_struct *t);
 		if (likely(!READ_ONCE((t)->trc_reader_special.b.need_qs)) &&	\
 		    likely(!___rttq_nesting)) {					\
 			rcu_trc_cmpxchg_need_qs((t), 0,	TRC_NEED_QS_CHECKED);	\
-		} else if (___rttq_nesting &&					\
+		} else if (___rttq_nesting && ___rttq_nesting != INT_MIN &&	\
 			   !READ_ONCE((t)->trc_reader_special.b.blocked)) {	\
 			rcu_tasks_trace_qs_blkd(t);				\
 		}								\
diff --git a/include/linux/rcupdate_trace.h b/include/linux/rcupdate_trace.h
index 6f9c358173989..9bc8cbb33340b 100644
--- a/include/linux/rcupdate_trace.h
+++ b/include/linux/rcupdate_trace.h
@@ -75,7 +75,7 @@ static inline void rcu_read_unlock_trace(void)
 	nesting = READ_ONCE(t->trc_reader_nesting) - 1;
 	barrier(); // Critical section before disabling.
 	// Disable IPI-based setting of .need_qs.
-	WRITE_ONCE(t->trc_reader_nesting, INT_MIN);
+	WRITE_ONCE(t->trc_reader_nesting, INT_MIN + nesting);
 	if (likely(!READ_ONCE(t->trc_reader_special.s)) || nesting) {
 		WRITE_ONCE(t->trc_reader_nesting, nesting);
 		return;  // We assume shallow reader nesting.