linux-kernel - Re: [PATCH 4/4] rcu: Use _full() API to debug synchronize

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Message-ID: <00900afe-ac4e-4362-a3f9-d65f2c9dcd9a@paulmck-laptop>
Date: Fri, 24 Jan 2025 07:49:24 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Uladzislau Rezki <urezki@...il.com>
Cc: Boqun Feng <boqun.feng@...il.com>, RCU <rcu@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Cheung Wall <zzqq0103.hey@...il.com>,
	Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 4/4] rcu: Use _full() API to debug synchronize_rcu()

On Fri, Jan 24, 2025 at 12:48:12PM +0100, Uladzislau Rezki wrote:
> 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@...il.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?

Why not?  Maybe separate the two differences with a colon.

Or maybe make a variant of poll_state_synchronize_rcu_full() that take
a char* argument, which uses the same value for the check and the string
to be output.

							Thanx, Paul