linux-kernel - Re: [PATCH v3 2/4] srcu: Add srcu_read_lock_fast_notrace() and srcu_read_unlock_fast

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Message-ID: <4ac56245-3185-414d-9ee1-2c4b4c0a9d5b@paulmck-laptop>
Date: Tue, 22 Jul 2025 15:34:52 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v3 2/4] srcu: Add srcu_read_lock_fast_notrace() and
 srcu_read_unlock_fast_notrace()

On Tue, Jul 22, 2025 at 06:11:00PM -0400, Joel Fernandes wrote:
> On Mon, Jul 21, 2025 at 09:24:31AM -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.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > ---
> >  include/linux/srcu.h | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> > 
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index 478c73d067f7d..7a692bf8f99b9 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -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)
> 
> Should these also be marked with 'notrace' attribute?
> 
> I am not sure what the precedent is, I do see a few examples of 'notrace' and
> 'inline' in the same function signature though.

Heh!!!

There are six instance of static-inline notrace functions, and eight
instances of static-inline non-notrace functions whose names contain
"_notrace", not counting the srcu_read_lock_fast_notrace() and
srcu_read_unlock_fast() functions currently under review.

My guess is that I should add "notrace" to handle the possible case
where the compiler declines to inline this function.  I will do this
on the next rebase unless I hear otherwise.

Steven, Mathieu, thoughts?

							Thanx, Paul

> Other than that one nit:
> Reviewed-by: Joel Fernandes <joelagnelf@...dia.com>
> 
> thanks,
> 
>  - Joel
> 
> 
> > +{
> > +	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.
> > @@ -394,6 +408,17 @@ 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.
> > -- 
> > 2.40.1
> >