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@xxxxxxxxx> > > > >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 > >
