linux-kernel - Re: Question WRT early IRQ/NMI entry code

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Message-ID: <20211130120511.GB599355@lothringen>
Date:   Tue, 30 Nov 2021 13:05:11 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Nicolas Saenz Julienne <nsaenzju@...hat.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        rcu@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Steven Rostedt <rostedt@...dmis.org>, paulmck@...nel.org,
        mtosatti <mtosatti@...hat.com>
Subject: Re: Question WRT early IRQ/NMI entry code

On Tue, Nov 30, 2021 at 12:28:41PM +0100, Nicolas Saenz Julienne wrote:
> Hi All,
> while going over the IRQ/NMI entry code I've found a small 'inconsistency':
> while in the IRQ entry path, we inform RCU of the context change *before*
> incrementing the preempt counter, the opposite happens for the NMI entry
> path. This applies to both arm64 and x86[1].
> 
> Actually, rcu_nmi_enter() — which is also the main RCU context switch function
> for the IRQ entry path — uses the preempt counter to verify it's not in NMI
> context. So it would make sense to assume all callers have the same updated
> view of the preempt count, which isn't true ATM.
> 
> I'm sure there an obscure/non-obvious reason for this, right?

At least I can't find an immediate reason for it either :-)

Thanks!