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 <paul...@kernel.org>
> > > > Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
> > > > Cc: Steven Rostedt <rost...@goodmis.org>
> > > > Cc: Sebastian Andrzej Siewior <bige...@linutronix.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 <joelagn...@nvidia.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
