Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sat, 22 Jul 2023 10:40:54 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Yun Levi <ppbuk5246@gmail.com>
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
Subject: Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on
 rcu_report_qs_rdp
Message-ID: <faf07eef-0a51-49f3-be48-0433952171ad@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20230721121534.44328-1-ppbuk5246@gmail.com>
 <05e98227-77f4-4918-8f8e-2170a158e350@paulmck-laptop>
 <CAM7-yPRc4Z0siaiWf+fK2GEfjPMq5UaY13F4o19rU6NNUS5fNg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAM7-yPRc4Z0siaiWf+fK2GEfjPMq5UaY13F4o19rU6NNUS5fNg@mail.gmail.com>
Precedence: bulk

On Sat, Jul 22, 2023 at 10:23:26AM +0100, Yun Levi wrote:
> 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?

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?

							Thanx, Paul

> > 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