Re: [PATCH v3 1/4] srcu: Move rcu_is_watching() checks to srcu_read_{,un}lock_fast()

[Joel Fernandes <joelagnelf@nvidia.com>]

On Mon, Jul 21, 2025 at 09:24:30AM -0700, Paul E. McKenney wrote:
> The rcu_is_watching() warnings are currently in the SRCU-tree
> implementations of __srcu_read_lock_fast() and __srcu_read_unlock_fast().
> However, this makes it difficult to create _notrace variants of
> srcu_read_lock_fast() and srcu_read_unlock_fast().  This commit therefore
> moves these checks to srcu_read_lock_fast(), srcu_read_unlock_fast(),
> srcu_down_read_fast(), and srcu_up_read_fast().
> 
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>

thanks,

 - Joel



> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  include/linux/srcu.h     | 4 ++++
>  include/linux/srcutree.h | 2 --
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index f179700fecafb..478c73d067f7d 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -275,6 +275,7 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *
>  {
>  	struct srcu_ctr __percpu *retval;
>  
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_fast().");
>  	srcu_check_read_flavor_force(ssp, SRCU_READ_FLAVOR_FAST);
>  	retval = __srcu_read_lock_fast(ssp);
>  	rcu_try_lock_acquire(&ssp->dep_map);
> @@ -295,6 +296,7 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *
>  static inline struct srcu_ctr __percpu *srcu_down_read_fast(struct srcu_struct *ssp) __acquires(ssp)
>  {
>  	WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi());
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_down_read_fast().");
>  	srcu_check_read_flavor_force(ssp, SRCU_READ_FLAVOR_FAST);
>  	return __srcu_read_lock_fast(ssp);
>  }
> @@ -389,6 +391,7 @@ static inline void srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_ct
>  	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
>  	srcu_lock_release(&ssp->dep_map);
>  	__srcu_read_unlock_fast(ssp, scp);
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast().");
>  }
>  
>  /**
> @@ -405,6 +408,7 @@ static inline void srcu_up_read_fast(struct srcu_struct *ssp, struct srcu_ctr __
>  	WARN_ON_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) && in_nmi());
>  	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST);
>  	__srcu_read_unlock_fast(ssp, scp);
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_up_read_fast().");
>  }
>  
>  /**
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index bf44d8d1e69ea..043b5a67ef71e 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -244,7 +244,6 @@ static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct
>  {
>  	struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
>  
> -	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_fast().");
>  	if (!IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
>  		this_cpu_inc(scp->srcu_locks.counter); /* Y */
>  	else
> @@ -275,7 +274,6 @@ static inline void __srcu_read_unlock_fast(struct srcu_struct *ssp, struct srcu_
>  		this_cpu_inc(scp->srcu_unlocks.counter);  /* Z */
>  	else
>  		atomic_long_inc(raw_cpu_ptr(&scp->srcu_unlocks));  /* Z */
> -	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_unlock_fast().");
>  }
>  
>  void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
> -- 
> 2.40.1
>