linux-kernel - Re: [PATCH v4 2/6] srcu: Add srcu_read_lock_fast_notrace() and srcu_read_unlock_fast

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Message-ID: <aIFLG91VhtjN8iaf@tardis.local>
Date: Wed, 23 Jul 2025 13:50:35 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Joel Fernandes <joelagnelf@...dia.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	bpf@...r.kernel.org
Subject: Re: [PATCH v4 2/6] srcu: Add srcu_read_lock_fast_notrace() and
 srcu_read_unlock_fast_notrace()

On Wed, Jul 23, 2025 at 01:27:56PM -0700, Paul E. McKenney wrote:
> This commit adds no-trace variants of the srcu_read_lock_fast() and
> srcu_read_unlock_fast() functions for tracing use.
> 
> [ paulmck: Apply notrace feedback from Joel Fernandes, Steven Rostedt, and Mathieu Desnoyers. ]
> 
> Link: https://lore.kernel.org/all/20250721162433.10454-1-paulmck@kernel.org
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Reviewed-by: Joel Fernandes <joelagnelf@...dia.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: <bpf@...r.kernel.org>
> ---
>  include/linux/srcu.h     | 30 ++++++++++++++++++++++++++++--
>  include/linux/srcutree.h |  5 +++--
>  2 files changed, 31 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index 478c73d067f7d..ec3b8e27d6c5a 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -271,7 +271,7 @@ static inline int srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp)
>   * where RCU is watching, that is, from contexts where it would be legal
>   * to invoke rcu_read_lock().  Otherwise, lockdep will complain.
>   */
> -static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *ssp) __acquires(ssp)
> +static inline struct srcu_ctr __percpu notrace *srcu_read_lock_fast(struct srcu_struct *ssp) __acquires(ssp)

Hmm.. am I missing something, why do we need ot make
srcu_read_lock_fast() notrace? I thought we only need those _notrace()
variants notrace?

Regards,
Boqun

>  {
>  	struct srcu_ctr __percpu *retval;
>  
> @@ -282,6 +282,20 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *
>  	return retval;
>  }
>  
> +/*
> + * Used by tracing, cannot be traced and cannot call lockdep.
> + * See srcu_read_lock_fast() for more information.
> + */
> +static inline struct srcu_ctr __percpu *srcu_read_lock_fast_notrace(struct srcu_struct *ssp)
> +	__acquires(ssp)
> +{
> +	struct srcu_ctr __percpu *retval;
> +
> +	srcu_check_read_flavor_force(ssp, SRCU_READ_FLAVOR_FAST);
> +	retval = __srcu_read_lock_fast(ssp);
> +	return retval;
> +}
> +
>  /**
>   * srcu_down_read_fast - register a new reader for an SRCU-protected structure.
>   * @ssp: srcu_struct in which to register the new reader.
> @@ -385,7 +399,8 @@ static inline void srcu_read_unlock(struct srcu_struct *ssp, int idx)
>   *
>   * Exit a light-weight SRCU read-side critical section.
>   */
> -static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
> +static inline void notrace
> +srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
>  	__releases(ssp)
>  {
>  	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
> @@ -394,6 +409,17 @@ static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ct
>  	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast().");
>  }
>  
> +/*
> + * Used by tracing, cannot be traced and cannot call lockdep.
> + * See srcu_read_unlock_fast() for more information.
> + */
> +static inline void srcu_read_unlock_fast_notrace(struct srcu_struct *ssp,
> +						 struct srcu_ctr __percpu *scp) __releases(ssp)
> +{
> +	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
> +	__srcu_read_unlock_fast(ssp, scp);
> +}
> +
>  /**
>   * srcu_up_read_fast - unregister a old reader from an SRCU-protected structure.
>   * @ssp: srcu_struct in which to unregister the old reader.
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index 043b5a67ef71e..4d2fee4d38289 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -240,7 +240,7 @@ static inline struct srcu_ctr __percpu *__srcu_ctr_to_ptr(struct srcu_struct *ss
>   * on architectures that support NMIs but do not supply NMI-safe
>   * implementations of this_cpu_inc().
>   */
> -static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct *ssp)
> +static inline struct srcu_ctr __percpu notrace *__srcu_read_lock_fast(struct srcu_struct *ssp)
>  {
>  	struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
>  
> @@ -267,7 +267,8 @@ static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct
>   * on architectures that support NMIs but do not supply NMI-safe
>   * implementations of this_cpu_inc().
>   */
> -static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
> +static inline void notrace
> +__srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ctr __percpu *scp)
>  {
>  	barrier();  /* Avoid leaking the critical section. */
>  	if (!IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
> -- 
> 2.40.1
> 
>