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Date:   Mon, 4 Dec 2023 20:23:01 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Boqun Feng <boqun.feng@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
        Frederic Weisbecker <frederic@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        John Ogness <john.ogness@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <longman@redhat.com>,
        Will Deacon <will@kernel.org>,
        Zqiang <qiang.zhang1211@gmail.com>
Subject: Re: [PATCH v2] srcu: Use try-lock lockdep annotation for NMI-safe
 access.
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 <ZV5Dhjcb9Jd_lk0O@boqun-archlinux>
 <20231130132729.niM5FJPc@linutronix.de>
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On Thu, Nov 30, 2023 at 02:27:29PM +0100, Sebastian Andrzej Siewior wrote:
> It is claimed that srcu_read_lock_nmisafe() NMI-safe. However it
> triggers a lockdep if used from NMI because lockdep expects a deadlock
> since nothing disables NMIs while the lock is acquired.
> 
> This is because commit f0f44752f5f61 ("rcu: Annotate SRCU's update-side
> lockdep dependencies") annotates synchronize_srcu() as a write lock
> usage. This helps to detect a deadlocks such as
> 	srcu_read_lock();
> 	synchronize_srcu();
> 	srcu_read_unlock();
> 
> The side effect is that the lock srcu_struct now has a USED usage in normal
> contexts, so it conflicts with a USED_READ usage in NMI. But this shouldn't
> cause a real deadlock because the write lock usage from synchronize_srcu() is a
> fake one and only used for read/write deadlock detection.
> 
> Use a try-lock annotation for srcu_read_lock_nmisafe() to avoid lockdep
> complains if used from NMI.
> 
> Fixes: f0f44752f5f61 ("rcu: Annotate SRCU's update-side lockdep dependencies")
> Link: https://lore.kernel.org/r/20230927160231.XRCDDSK4@linutronix.de
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Queued for v6.9 along with further review and testing, thank you both!

							Thanx, Paul

> ---
>  include/linux/rcupdate.h | 6 ++++++
>  include/linux/srcu.h     | 2 +-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index f7206b2623c98..31d523c4e0893 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -301,6 +301,11 @@ static inline void rcu_lock_acquire(struct lockdep_map *map)
>  	lock_acquire(map, 0, 0, 2, 0, NULL, _THIS_IP_);
>  }
>  
> +static inline void rcu_try_lock_acquire(struct lockdep_map *map)
> +{
> +	lock_acquire(map, 0, 1, 2, 0, NULL, _THIS_IP_);
> +}
> +
>  static inline void rcu_lock_release(struct lockdep_map *map)
>  {
>  	lock_release(map, _THIS_IP_);
> @@ -315,6 +320,7 @@ int rcu_read_lock_any_held(void);
>  #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
>  
>  # define rcu_lock_acquire(a)		do { } while (0)
> +# define rcu_try_lock_acquire(a)	do { } while (0)
>  # define rcu_lock_release(a)		do { } while (0)
>  
>  static inline int rcu_read_lock_held(void)
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index 127ef3b2e6073..236610e4a8fa5 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -229,7 +229,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp
>  
>  	srcu_check_nmi_safety(ssp, true);
>  	retval = __srcu_read_lock_nmisafe(ssp);
> -	rcu_lock_acquire(&ssp->dep_map);
> +	rcu_try_lock_acquire(&ssp->dep_map);
>  	return retval;
>  }
>  
> -- 
> 2.43.0