Re: [PATCH 06/13] srcutree: Make init_srcu_struct_atomic() prevent transition to big

[Kunwu Chan <kunwu.chan@gmail.com>]

On Tue, 8 Sep 2026 16:36:11 -0700 "Paul E. McKenney" <paulmck@kernel.org> wrote:

> On Mon, Sep 07, 2026 at 03:58:22PM +0800, Kunwu Chan wrote:
> > From: Kunwu Chan <kunwu.chan@gmail.com>
> > 
> > The is_atomic parameter of init_srcu_struct_fields() exists so that
> > atomic SRCU never transitions to big, but neither
> > init_srcu_struct_atomic() nor its lockdep counterpart
> > __init_srcu_struct_atomic() sets it.
> > 
> > On systems where srcutree.convert_to_big selects SRCU_SIZING_INIT,
> > this needlessly allocates a full srcu_node combining tree for any
> > dynamically initialized atomic srcu_struct, despite atomic SRCU
> > having neither callbacks nor srcu_barrier() operations.
> > 
> > Pass true from both atomic entry points, adding an is_atomic parameter
> > to __init_srcu_struct_common().
> > 
> > Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
> 
> Given that we have this commit, is this patch needed?
> 
> 4e01d5320a2f ("srcutree: Suppress to-big transition for atomic SRCU")
> 
> If so, please tell me what I am missing.

Yes, this patch is still needed. The is_atomic mechanism is there, but 
the atomic initialization paths still pass false.

In particular, init_srcu_struct_atomic() passes false directly, while the 
lockdep path goes through __init_srcu_struct_common(), which also passes false.

This patch makes both atomic entry points propagate true.

Thanks,
KunWu

> 
> 						Thanx, Paul
> 
> > ---
> >  kernel/rcu/srcutree.c | 15 ++++++++-------
> >  1 file changed, 8 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 2af36db37fa9..01f224a56b41 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -305,26 +305,27 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static, bool
> >  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> >  
> >  static int
> > -__init_srcu_struct_common(struct srcu_struct *ssp, const char *name, struct lock_class_key *key)
> > +__init_srcu_struct_common(struct srcu_struct *ssp, const char *name,
> > +			  struct lock_class_key *key, bool is_atomic)
> >  {
> >  	/* Don't re-initialize a lock while it is held. */
> >  	debug_check_no_locks_freed((void *)ssp, sizeof(*ssp));
> >  	lockdep_init_map(&ssp->dep_map, name, key, 0);
> > -	return init_srcu_struct_fields(ssp, false, false);
> > +	return init_srcu_struct_fields(ssp, false, is_atomic);
> >  }
> >  
> >  int init_srcu_struct_lockdep(struct srcu_struct *ssp, const char *name,
> >  			     struct lock_class_key *key)
> >  {
> >  	ssp->srcu_reader_flavor = 0;
> > -	return __init_srcu_struct_common(ssp, name, key);
> > +	return __init_srcu_struct_common(ssp, name, key, false);
> >  }
> >  EXPORT_SYMBOL_GPL(init_srcu_struct_lockdep);
> >  
> >  int __init_srcu_struct_fast(struct srcu_struct *ssp, const char *name, struct lock_class_key *key)
> >  {
> >  	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_FAST;
> > -	return __init_srcu_struct_common(ssp, name, key);
> > +	return __init_srcu_struct_common(ssp, name, key, false);
> >  }
> >  EXPORT_SYMBOL_GPL(__init_srcu_struct_fast);
> >  
> > @@ -332,14 +333,14 @@ int __init_srcu_struct_fast_updown(struct srcu_struct *ssp, const char *name,
> >  				   struct lock_class_key *key)
> >  {
> >  	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_FAST_UPDOWN;
> > -	return __init_srcu_struct_common(ssp, name, key);
> > +	return __init_srcu_struct_common(ssp, name, key, false);
> >  }
> >  EXPORT_SYMBOL_GPL(__init_srcu_struct_fast_updown);
> >  
> >  int __init_srcu_struct_atomic(struct srcu_struct *ssp, const char *name, struct lock_class_key *key)
> >  {
> >  	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_ATOMIC;
> > -	return __init_srcu_struct_common(ssp, name, key);
> > +	return __init_srcu_struct_common(ssp, name, key, true);
> >  }
> >  EXPORT_SYMBOL_GPL(__init_srcu_struct_atomic);
> >  
> > @@ -414,7 +415,7 @@ EXPORT_SYMBOL_GPL(init_srcu_struct_fast_updown);
> >  int init_srcu_struct_atomic(struct srcu_struct *ssp)
> >  {
> >  	ssp->srcu_reader_flavor = SRCU_READ_FLAVOR_ATOMIC;
> > -	return init_srcu_struct_fields(ssp, false, false);
> > +	return init_srcu_struct_fields(ssp, false, true);
> >  }
> >  EXPORT_SYMBOL_GPL(init_srcu_struct_atomic);
> >  
> > -- 
> > 2.43.0
> > 
>