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

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Message-ID: <20230928080556.GE9829@noisy.programming.kicks-ass.net>
Date:   Thu, 28 Sep 2023 10:05:56 +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:

> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index e85b5ad3e206..1af8d44e5eb4 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -5727,8 +5727,9 @@ void lock_acquire(struct lockdep_map *lock, unsigned int subclass,
>                 return;
> 
>         if (unlikely(!lockdep_enabled())) {
> +               /* Only do NMI context checking if it's a check lock */
>                 /* XXX allow trylock from NMI ?!? */
> -               if (lockdep_nmi() && !trylock) {
> +               if (check && lockdep_nmi() && !trylock) {
>                         struct held_lock hlock;
> 
>                         hlock.acquire_ip = ip;
> 
> Peter, thoughts?
> 

I think I prefer the trylock one. Fundamentally trylock conveys the 'we
wont block' thing. Making 'lock' sometimes work for NMI is just
confusing.