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

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Date:   Thu, 28 Sep 2023 07:54:10 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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?

One thing though, before the commit, srcu_read_lock() already has an
rcu_lock_acquire() annotation which eventually calls lock_acquire()
which has a tracepoint in it.

Regards,
Boqun