On Thu, Jul 14, 2022 at 01:53:20AM +0000, Zhang, Qiang1 wrote:
>
> On Tue, Jul 12, 2022 at 04:26:06PM +0800, Zqiang wrote:
> > When the rcu_scheduler_active variable is RCU_SCHEDULER_INACTIVE and not
> > yet converted to RCU_SCHEDULER_INIT, there is only idle task, any legal
> > call synchronize_rcu_tasks_generic() is a quiescent state. this commit
> > make synchronize_rcu_tasks_generic() no-ops when the rcu_scheduler_active
> > variable is RCU_SCHEDULER_INACTIVE.
> >
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> > ---
>
> >It looks like this would be a good way to provide early boot access
> >to synchronize_rcu_tasks(), synchronize_rcu_tasks_rude(), and
> >synchronize_rcu_tasks_trace().
> >
> >But do we really need early boot access to these functions? As in has
> >the below WARN_ON() actually triggered?
>
> when the rcu_scheduler_active variable is RCU_SCHEDULER_INACTIVE,
> invoke synchronize_rcu_tasks_generic(), in addition to triggering a warning,
> also need to make it return directly, if not, the rcu_tasks_one_gp() will be
> called directly, but due to the rtp structure's -> pregp_func is not initialized,
> A null pointer bug will appear.
>
> But like said, I don't see the need to call synchronize_rcu_tasks_generic() on
> early booting. maybe this change is not necessary.
Not yet, anyway. And adding this would require more testing.
However, if the current warning does trigger, and the caller has a
legitimate reason for invoking this function so early, please remember
this patch. ;-)
Thanx, Paul
> Thanks
> Zqiang
>
> >
> >And if it has triggered, and in a context that means that these functions
> >really are needed during early boot, how should the testing strategy
> >change to test these at the relevant portions of the boot sequence?
> >
> >>From what I know, hitting these during early boot would indicate that
> >something was removing a trace during early boot, and I know of no way
> >to make that happen. Hence my skepticism. ;-)
> >
> >But *if* this was really needed, this looks to be a reasonable way to
> >implement it.
> >
> > Thanx, Paul
>
> > kernel/rcu/tasks.h | 5 +++--
> > 1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index 469bf2a3b505..0237e765c28e 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -560,8 +560,9 @@ static int __noreturn rcu_tasks_kthread(void *arg)
> > static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
> > {
> > /* Complain if the scheduler has not started. */
> > - WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
> > - "synchronize_rcu_tasks called too soon");
> > + if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
> > + "synchronize_rcu_tasks called too soon"))
> > + return;
> >
> > // If the grace-period kthread is running, use it.
> > if (READ_ONCE(rtp->kthread_ptr)) {
> > --
> > 2.25.1
> >
