linux-kernel - Re: [PATCH 02/11] rcu: Kill rnp->ofl_seq and use only rcu_state.ofl

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Date:   Thu, 09 Dec 2021 18:43:30 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        hushiyuan@...wei.com, luolongjun@...wei.com, hejingxian@...wei.com
Subject: Re: [PATCH 02/11] rcu: Kill rnp->ofl_seq and use only
 rcu_state.ofl_lock for exclusion

On Fri, 2021-12-10 at 00:01 +0530, Neeraj Upadhyay wrote:
> > @@ -4246,11 +4249,11 @@ void rcu_cpu_starting(unsigned int cpu)
> >    
> >        rnp = rdp->mynode;
> >        mask = rdp->grpmask;
> > -     WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
> > -     WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
> > +     local_irq_save(seq_flags);
> > +     arch_spin_lock(&rcu_state.ofl_lock);
> >        rcu_dynticks_eqs_online();
> >        smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
> 
> Can we drop this smp_mb(),as arch_spin_lock(&rcu_state.ofl_lock) 
> provides the ordering now?

Yes, thanks.

> > -     raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > +     raw_spin_lock_rcu_node(rnp);
> >        WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
> >        newcpu = !(rnp->expmaskinitnext & mask);
> >        rnp->expmaskinitnext |= mask;
> > @@ -4269,9 +4272,8 @@ void rcu_cpu_starting(unsigned int cpu)
> >        } else {
> >                raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> 
> 'flags' is uninitialized now?

Ah yes, that suffered from the fact that saving the flags is pointless
because we know we already disabled interrupts... but
rcu_report_qs_rnp() *really* wants to be given some flags to restore.

Will fix that too; thanks again.

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