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Date:   Thu, 20 Oct 2022 15:16:04 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, quic_neeraju@quicinc.com,
        rcu@vger.kernel.org, Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH 1/3] srcu: Warn when NMI-unsafe API is used in NMI
Message-ID: <20221020221604.GW5600@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20221013172244.1099010-1-frederic@kernel.org>
 <20221013172244.1099010-2-frederic@kernel.org>
 <Y0nmcH0SktDdonyW@google.com>
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On Fri, Oct 14, 2022 at 10:45:04PM +0000, Joel Fernandes wrote:
> On Thu, Oct 13, 2022 at 07:22:42PM +0200, Frederic Weisbecker wrote:
> > Using the NMI-unsafe reader API from within NMIs is very likely to be
> > buggy for three reasons:
> > 
> > 1) NMIs aren't strictly re-entrant (a pending nested NMI will execute
> >    at the end of the current one) so it should be fine to use a
> >    non-atomic increment here. However breakpoints can still interrupt
> >    NMIs and if a breakpoint callback has a reader on that same ssp, a
> >    racy increment can happen.
> > 
> > 2) If the only reader site for a given ssp is in an NMI, RCU is definetly
> 								  definitely
> >    a better choice over SRCU.
> 
> Just checking - because NMI are by definition not-preemptibe, so SRCU over
> RCU doesn't make much sense right?

Agreed.  But you never know...

> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

I will apply on the next rebase (after today's rebase), thank you!

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> > 
> > 3) Because of the previous reason (2), an ssp having an SRCU read side
> >    critical section in an NMI is likely to have another one from a task
> >    context.
> > 
> > For all these reasons, warn if an nmi unsafe reader API is used from an
> > NMI.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > ---
> >  kernel/rcu/srcutree.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index c54142374793..8b7ef1031d89 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -642,6 +642,8 @@ static void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe)
> >  
> >  	if (!IS_ENABLED(CONFIG_PROVE_RCU))
> >  		return;
> > +	/* NMI-unsafe use in NMI is a bad sign */
> > +	WARN_ON_ONCE(!nmi_safe && in_nmi());
> >  	sdp = raw_cpu_ptr(ssp->sda);
> >  	old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety);
> >  	if (!old_nmi_safe_mask) {
> > -- 
> > 2.25.1
> >