linux-kernel - Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on rcu_report_qs

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Message-ID: <31070aab-8665-44c4-8950-0631a777ef44@paulmck-laptop>
Date:   Sat, 22 Jul 2023 15:39:42 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Yun Levi <ppbuk5246@...il.com>
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, osh@...htriplett.org, boqun.feng@...il.com,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, qiang.zhang1211@...il.com,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on
 rcu_report_qs_rdp

On Sat, Jul 22, 2023 at 09:14:28PM +0100, Yun Levi wrote:
> Hi Paul.
> 
> > Suppose that the scheduler-clock interrupt invoking rcu_sched_clock_irq()
> > happened just before the lock was acquired in rcu_report_qs_rdp().
> > Suppose further that the RCU grace-period kthread started a new grace
> > period just before that interrupt occurred.  Then mightn't that interrupt
> > notice the new grace period and set ->cpu_no_qs.b.norm to true before
> > fully returning?
> 
> IIUC, RCU grace-period kthread couldn't start new grace period
> because the interrupted cpu don't report qs to rnp  via rcu_report_qs_rdp.
> That situation is listened like new gp could be started thou all cpus
> doesn't enter yet.
> That's is the reason why it's better to use WARN_ON_ONCE as you suggest
> to notice if the buggy situation happens

And try testing with CONFIG_RCU_STRICT_GRACE_PERIOD=y and CONFIG_PREEMPT=n.
Though there might be better Kconfig options to use.  Those two come
immediately to mind.

> Am I missing something or wrong?

I cannot see into your head, so I cannot say.

But one critical piece is that softirq handlers, including the RCU_SOFTIRQ
handler rcu_core_si(), can be invoked upon return from interrupts.
Another critical piece is that if a CPU is idle during any part of a
grace period, the grace-period kthread can report a quiescent state on
its behalf.

Does that help?

							Thanx, Paul