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

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Message-ID: <20230928063350.KveBSZGg@linutronix.de>
Date:   Thu, 28 Sep 2023 08:33:50 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 2023-09-27 23:06:09 [-0700], Boqun Feng wrote:
> SRCU only has read lock usage from lockdep PoV, but after that commit,
> we annotate synchronize_srcu() as a write lock usage, so that we can
> detect deadlocks between *normal* srcu_read_lock() and
> synchronize_srcu(), however the side effect is now SRCU has a write lock
> usage from lockdep PoV.

Ach. There is a write annotation for SRCU and RCU has none. Okay that
explains it.

> Actually in the above commit, I explicitly leave
> srcu_read_lock_nmisafe() alone since its locking rules may be different
> compared to srcu_read_lock(). In lockdep terms, srcu_read_lock_nmisafe()
> is a !check read lock and srcu_read_lock() is a check read lock.

This was on v6.6-rc3 so it has the commit f0f44752f5f61 ("rcu: Annotate
SRCU's update-side lockdep dependencies").

>                                                                  Maybe
> instead of using the trylock trick, we change lockdep to igore !check
> locks for NMI context detection? Untested code as below:

Just tested, no splat for the SRCU-in-NMI usage.

Sebastian