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

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Message-ID: <20230928152942.GE27245@noisy.programming.kicks-ass.net>
Date:   Thu, 28 Sep 2023 17:29:42 +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 Thu, Sep 28, 2023 at 07:54:10AM -0700, Boqun Feng wrote:
> On Thu, Sep 28, 2023 at 10:09:00AM +0200, Peter Zijlstra wrote:
> > 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.
> > 
> 
> Interesting
> 
> > Was this considered and intended?
> > 
> 
> No, I don't think I have considered this before, I think I may still
> miss something here, maybe you or Paul can provide an example for such
> a case?

The whole trace_.*_rcuidle() machinery. Which I thought I had fully
eradicated, but apparently still exists (with *one* user) :-/

Search for rcuidle in include/linux/tracepoint.h

Also, git grep trace_.*_rcuidle