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From:   Yun Levi <ppbuk5246@gmail.com>
Date:   Sat, 22 Jul 2023 21:14:28 +0100
Message-ID: <CAM7-yPTrHN1xPXWjSUrJeTEOuy78DpmL8ytUY+a4ZOekiAbnZg@mail.gmail.com>
Subject: Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on rcu_report_qs_rdp
To:     paulmck@kernel.org
Cc:     frederic@kernel.org, quic_neeraju@quicinc.com,
        joel@joelfernandes.org, osh@joshtriplett.org, boqun.feng@gmail.com,
        rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        jiangshanlai@gmail.com, qiang.zhang1211@gmail.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org
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Precedence: bulk

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

Am I missing something or wrong?

Thanks.

--------
Sincerely,
Levi.