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: <807686da-42af-4196-bd8e-b763f073f104@paulmck-laptop>
Date: Wed, 23 Jul 2025 09:28:57 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Joel Fernandes <joelagnelf@...dia.com>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org,
	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 Wed, Jul 23, 2025 at 11:23:33AM -0400, Mathieu Desnoyers wrote:
> On 2025-07-22 18:34, Paul E. McKenney wrote:
> > 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?
> 
> AFAIR, the always_inline gcc attribute takes care of making sure the
> notrace is not needed in addition.
> 
> So I suspect that kernel APIs need to abide by the following rules
> to be usable by instrumentation code:
> 
> - if it's a function call, have a notrace attribute.
> - if it's an inline (which the compiler may decide to implement as a
>   function call), have a notrace attribute.
> - if it's an __always_inline, then there is no way the compiler can
>   possibly make it a function call, so the notrace would be useless
>   there.
> 
> So you may want to choose for either:
> 
> - inline notrace, or
> - __always_inline
> 
> Depending on how much freedom you want to grant the compiler with
> respect to its inlining practices.

In this case, I will (uncharacteristically) grant the compiler some
freedom.  And so notrace it is!  ;-)

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> 
> > 
> > 							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
> > > > 
> 
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com