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. o CPU 1 starts a new RCU Tasks Trace grace period. o CPU 0 reaches the newly added rcu_note_voluntary_context_switch(). 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 >
