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

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Message-ID: <CAM7-yPRc4Z0siaiWf+fK2GEfjPMq5UaY13F4o19rU6NNUS5fNg@mail.gmail.com>
Date:   Sat, 22 Jul 2023 10:23:26 +0100
From:   Yun Levi <ppbuk5246@...il.com>
To:     paulmck@...nel.org
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

Hi, Paul.

Thanks for looking into this :)


> Except that rcu_report_qs_rdp() is invoked with interrupts enabled,
> which means that there is some possibility of state changes up to the
> raw_spin_lock_irqsave_rcu_node(rnp, flags) statement.
>
> So, did you check whether RCU's interrupt paths change this state?

In my narrow view,
only a new gp started, cpu_no_qs.b.norm changes as true in the path of
rcu_sched_clock_irq.
But in that case, rcu_report_qs_rdp isn't called.

Did I understand your question well and are there any missed paths I didn't see?

> Why not start with something like this?
>
>         if (!WARN_ON_ONCE(!rdp->cpu_no_qs.b.norm) ||
>             rdp->gp_seq != rnp->gp_seq || rdp->gpwrap) {
>

Yes. but with different message