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

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

On Tue, Jul 19, 2022 at 04:34:58AM +0000, Zhang, Qiang1 wrote:
> 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 😊.

I do see both copies, thank you.  And yes, I am deferring the wordsmithing
to  a time when I am awake.  ;-)

							Thanx, Paul

> 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
> > >