RE: [PATCH v2] rcu-tasks: Make RCU Tasks Trace checking for userspace execution

["Zhang, Qiang1" <qiang1.zhang@intel.com>]

On Mon, Jul 18, 2022 at 11:54:53PM +0000, Zhang, Qiang1 wrote:
> On Mon, Jul 18, 2022 at 08:16:10AM +0800, Zqiang wrote:
> > For RCU tasks trace, the userspace execution is also a valid 
> > quiescent state, if the task is in userspace, the 
> > ->trc_reader_nesting should be zero and if the 
> > ->trc_reader_special.b.need_qs is not set, set the tasks 
> > ->trc_reader_special.b.need_qs is TRC_NEED_QS_CHECKED, this cause grace-period kthread remove it from holdout list if it remains here.
> > 
> > This commit add rcu_tasks_trace_qs() to rcu_flavor_sched_clock_irq() 
> > when the kernel built with no PREEMPT_RCU.
> > 
> > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> >
> >The looks plausible to me, but can you tell me how this avoids the 
> >following sequence of events?
> >
> >o	CPU 0 takes a scheduling-clock interrupt.  Just before this
> >	point CPU 0 was running in user context, thus as you say
> >	should not be in an RCU Tasks quiescent state.
> >
> >o	CPU 0 enters an RCU Tasks Trace read-side critical section.
>                
>  if I understand correctly, you mean that CPU0 enters an RCU Tasks 
> Trace  read-side critical section in scheduling-clock interrupt context.
>
>Exactly, as might happen if one of the functions in the scheduling-clock interrupt hander were traced/instrumented.
>
> >o	CPU 1 starts a new RCU Tasks Trace grace period.
> 
> The grace period kthread will scan running tasks on each CPU, The 
> tasks currently running on CPU0 will be recorded in the holdout list.
>
>Yes, very good.
>
> >o	CPU 0 reaches the newly added rcu_note_voluntary_context_switch().
> 
> In this time, if CPU0 still in RCU Tasks Trace read-side critical 
> section, the tasks which running on CPU0 will insert CPU0 blocked 
> list. when this tasks exit RCU Tasks Trace read-side critical section, this task will remove from CPU0 block list.
> 
> Did I understand the scenario described above correctly?
>
>Looks like it to me.
>
>Could you please resend the patch with this explained in the commit log?  Possibly for the benefit of your future self.  ;-)
>

Hi Paul, 

I have resent v3 again, but maybe still need your wording 😊.

Thanks
Zqiang

>							Thanx, Paul
>
> Thanks
> Zqiang
> 
> >
> >	Except that the quiescent state implied by userspace execution
> >	was before the new grace period, and thus does not apply to it.
> >
> >(Yes, I know, if this is a bug in this patch, the bug already exists 
> >due to the call in rcu_flavor_sched_clock_irq() for !PREEMPT kernels, 
> >but if this change is safe, it should be possible to explain why.)
> >
> >							Thanx, Paul
> >
> > ---
> >  v1->v2:
> >  Fix build error due to undeclared rcu_tasks_trace_qs(), note in 
> > no-PREEMPT_RCU  kernel, the RCU Tasks is replaced by RCU, so 
> > rcu_note_voluntary_context_switch()
> >  only include rcu_tasks_trace_qs().
> > 
> >  kernel/rcu/tree_plugin.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h 
> > index 4152816dd29f..5fb0b2dd24fd 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -976,7 +976,7 @@ static void rcu_flavor_sched_clock_irq(int user)
> >  		 * neither access nor modify, at least not while the
> >  		 * corresponding CPU is online.
> >  		 */
> > -
> > +		rcu_note_voluntary_context_switch(current);
> >  		rcu_qs();
> >  	}
> >  }
> > --
> > 2.25.1
> >