linux-kernel - Re: [PATCH 15/19] srcu: Create an SRCU-fast-updown API

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Message-ID: <e166df6b-1d04-4962-854d-8c88010bff5c@paulmck-laptop>
Date: Tue, 4 Nov 2025 08:49:27 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Akira Yokosawa <akiyks@...il.com>
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
	kernel-team@...a.com, linux-kernel@...r.kernel.org,
	peterz@...radead.org, rcu@...r.kernel.org, rostedt@...dmis.org
Subject: Re: [PATCH 15/19] srcu: Create an SRCU-fast-updown API

On Tue, Nov 04, 2025 at 04:00:27PM +0900, Akira Yokosawa wrote:
> Hi Paul,
> 
> Minor nitpicks in kernel-doc comment of srcu_read_lock_fast_updown().
> 
> On Sun,  2 Nov 2025 13:44:32 -0800, Paul E. McKenney wrote:
> > This commit creates an SRCU-fast-updown API, including
> > DEFINE_SRCU_FAST_UPDOWN(), DEFINE_STATIC_SRCU_FAST_UPDOWN(),
> > __init_srcu_struct_fast_updown(), init_srcu_struct_fast_updown(),
> > srcu_read_lock_fast_updown(), srcu_read_unlock_fast_updown(),
> > __srcu_read_lock_fast_updown(), and __srcu_read_unlock_fast_updown().
> > 
> > These are initially identical to their SRCU-fast counterparts, but both
> > SRCU-fast and SRCU-fast-updown will be optimized in different directions
> > by later commits.  SRCU-fast will lack any sort of srcu_down_read() and
> > srcu_up_read() APIs, which will enable extremely efficient NMI safety.
> > For its part, SRCU-fast-updown will not be NMI safe, which will enable
> > reasonably efficient implementations of srcu_down_read_fast() and
> > srcu_up_read_fast().
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Andrii Nakryiko <andrii@...nel.org>
> > Cc: Alexei Starovoitov <ast@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: <bpf@...r.kernel.org>
> > ---
> >  include/linux/srcu.h     | 77 +++++++++++++++++++++++++++++++++++++---
> >  include/linux/srcutiny.h | 16 +++++++++
> >  include/linux/srcutree.h | 55 ++++++++++++++++++++++++++--
> >  kernel/rcu/srcutree.c    | 39 +++++++++++++++++---
> >  4 files changed, 176 insertions(+), 11 deletions(-)
> > 
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index 1dd6812aabe7..1fbf475eae5e 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> 
> [...]
> 
> > @@ -305,6 +315,46 @@ static inline struct srcu_ctr __percpu *srcu_read_lock_fast(struct srcu_struct *
> >  	return retval;
> >  }
> >  
> > +/**
> > + * srcu_read_lock_fast_updown - register a new reader for an SRCU-fast-updown structure.
> > + * @ssp: srcu_struct in which to register the new reader.
> > + *
> > + * Enter an SRCU read-side critical section, but for a light-weight
> > + * smp_mb()-free reader.  See srcu_read_lock() for more information.
> > + * This function is compatible with srcu_down_read_fast(), but is not
> > + * NMI-safe.
> > + *
> > + * For srcu_read_lock_fast_updown() to be used on an srcu_struct
> > + * structure, that structure must have been defined using either
> > + * DEFINE_SRCU_FAST_UPDOWN() or DEFINE_STATIC_SRCU_FAST_UPDOWN() on the one
> > + * hand or initialized with init_srcu_struct_fast_updown() on the other.
> > + * Such an srcu_struct structure cannot be passed to any non-fast-updown
> > + * variant of srcu_read_{,un}lock() or srcu_{down,up}_read().  In kernels
> > + * built with CONFIG_PROVE_RCU=y, () will complain bitterly if you ignore
> > + * this * restriction.
> 
> Probably,
> 
>  * built with CONFIG_PROVE_RCU=y, __srcu_check_read_flavor() will complain
>  * bitterly if you ignore this restriction.
> 
> ??
> 
> > + *
> > + * Grace-period auto-expediting is disabled for SRCU-fast-updown
> > + * srcu_struct structures because SRCU-fast-updown expedited grace periods
> > + * invoke synchronize_rcu_expedited(), IPIs and all.  If you need expedited
> > + * SRCU-fast-updown grace periods, use synchronize_srcu_expedited().
> > + *
> > + * The srcu_read_lock_fast_updown() function can be invoked only from
> > + those contexts where RCU is watching, that is, from contexts where
> > + it would be legal to invoke rcu_read_lock().  Otherwise, lockdep will
> > + complain.
> 
> kernel-doc (script) complains:
> 
> Warning: include/linux/srcu.h:341 bad line:  those contexts where RCU is watching, that is, from contexts where
> Warning: include/linux/srcu.h:342 bad line:  it would be legal to invoke rcu_read_lock().  Otherwise, lockdep will
> Warning: include/linux/srcu.h:343 bad line:  complain.
> 
> Leading "* "s are missing.

Good eyes on both, will fix, thank you!!!

							Thanx, Pual

>         Thanks, Akira
> 
> > + */
> > +static inline struct srcu_ctr __percpu *srcu_read_lock_fast_updown(struct srcu_struct *ssp)
> > +__acquires(ssp)
> > +{
> > +	struct srcu_ctr __percpu *retval;
> > +
> > +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_fast_updown().");
> > +	srcu_check_read_flavor(ssp, SRCU_READ_FLAVOR_FAST_UPDOWN);
> > +	retval = __srcu_read_lock_fast_updown(ssp);
> > +	rcu_try_lock_acquire(&ssp->dep_map);
> > +	return retval;
> > +}
> > +
> >  /*
> >   * Used by tracing, cannot be traced and cannot call lockdep.
> >   * See srcu_read_lock_fast() for more information.
> [...]