linux-kernel - Re: [RFC PATCH] srcu: Use try-lock lockdep annotation for NMI-safe access.

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Message-ID: <20230928080900.GF9829@noisy.programming.kicks-ass.net>
Date:   Thu, 28 Sep 2023 10:09:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        John Ogness <john.ogness@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>,
        Will Deacon <will@...nel.org>,
        Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [RFC PATCH] srcu: Use try-lock lockdep annotation for NMI-safe
 access.

On Wed, Sep 27, 2023 at 11:06:09PM -0700, Boqun Feng wrote:

> I think this is a "side-effect" of commit f0f44752f5f6 ("rcu: Annotate
> SRCU's update-side lockdep dependencies"). In verify_lock_unused(), i.e.
> the checking for NMI lock usages, the logic is that

I think I'm having a problem with this commit -- that is, by adding
lockdep you're adding tracepoint, which rely on RCU being active.

The result is that SRCU is now no longer usable from !RCU regions.

Was this considered and intended?