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Date:   Fri, 14 Oct 2022 22:45:04 +0000
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     "Paul E . McKenney" <paulmck@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: <Y0nmcH0SktDdonyW@google.com>
References: <20221013172244.1099010-1-frederic@kernel.org>
 <20221013172244.1099010-2-frederic@kernel.org>
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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?

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

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
>