Re: [PATCH 4/4] rcu: Use _full() API to debug synchronize_rcu()

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Jan 23, 2025 at 01:52:57PM -0800, Paul E. McKenney wrote:
> On Thu, Jan 23, 2025 at 07:58:28PM +0100, Uladzislau Rezki (Sony) wrote:
> > Switch for using of get_state_synchronize_rcu_full() and
> > poll_state_synchronize_rcu_full() pair for debug a normal
> > synchronize_rcu() call.
> > 
> > Just using "not" full APIs to identify if a grace period
> > is passed or not might lead to a false kernel splat.
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >  include/linux/rcupdate_wait.h | 4 ++++
> >  kernel/rcu/tree.c             | 8 +++-----
> >  2 files changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
> > index f9bed3d3f78d..a16fc2a9a7d7 100644
> > --- a/include/linux/rcupdate_wait.h
> > +++ b/include/linux/rcupdate_wait.h
> > @@ -16,6 +16,10 @@
> >  struct rcu_synchronize {
> >  	struct rcu_head head;
> >  	struct completion completion;
> > +#ifdef CONFIG_PROVE_RCU
> > +	/* This is for testing. */
> > +	struct rcu_gp_oldstate oldstate;
> > +#endif
> >  };
> >  void wakeme_after_rcu(struct rcu_head *head);
> >  
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 2795d6b5109c..0ae90089ef09 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1612,12 +1612,10 @@ static void rcu_sr_normal_complete(struct llist_node *node)
> >  {
> >  	struct rcu_synchronize *rs = container_of(
> >  		(struct rcu_head *) node, struct rcu_synchronize, head);
> > -	unsigned long oldstate = (unsigned long) rs->head.func;
> >  
> >  	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
> > -		!poll_state_synchronize_rcu(oldstate),
> > -		"A full grace period is not passed yet: %lu",
> > -		rcu_seq_diff(get_state_synchronize_rcu(), oldstate));
> > +		!poll_state_synchronize_rcu_full(&rs->oldstate),
> > +		"A full grace period is not passed yet!\n");
> 
> Looks good, but why not also continue printing out the required
> grace-period sequence number?  Yes, there would need to be helper
> sprintf()-style functions to paper over the difference between Tiny RCU
> and Tree RCU.  ;-)
> 
Uhh :) Do we have rcu_seq_diff() for a _full() API? Looks like not :)

It contains both, rgos_norm and rgos_exp! Take a delta of both?

--
Uladzislau Rezki