Re: [PATCH v2] rcu/segcblist: Add debug checks for segment lengths

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Nov 18, 2020 at 07:52:23PM -0800, Paul E. McKenney wrote:
> On Wed, Nov 18, 2020 at 12:13:35PM -0800, Paul E. McKenney wrote:
> > On Wed, Nov 18, 2020 at 11:15:41AM -0500, Joel Fernandes (Google) wrote:
> > > After rcu_do_batch(), add a check for whether the seglen counts went to
> > > zero if the list was indeed empty.
> > > 
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > 
> > Queued for testing and further review, thank you!
> 
> FYI, the second of the two checks triggered in all four one-hour runs of
> TREE01, all four one-hour runs of TREE04, and one of the four one-hour
> runs of TREE07.  This one:
> 
> WARN_ON_ONCE(count != 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
> 
> That is, there are callbacks in the list, but the sum of the segment
> counts is nevertheless zero.  The ->nocb_lock is held.
> 
> Thoughts?

FWIW, TREE01 reproduces it very quickly compared to the other two
scenarios, on all four run, within five minutes.

							Thanx, Paul

> > > ---
> > > v1->v2: Added more debug checks.
> > > 
> > >  kernel/rcu/rcu_segcblist.c | 12 ++++++++++++
> > >  kernel/rcu/rcu_segcblist.h |  3 +++
> > >  kernel/rcu/tree.c          |  2 ++
> > >  3 files changed, 17 insertions(+)
> > > 
> > > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> > > index 5059b6102afe..6e98bb3804f0 100644
> > > --- a/kernel/rcu/rcu_segcblist.c
> > > +++ b/kernel/rcu/rcu_segcblist.c
> > > @@ -94,6 +94,18 @@ static long rcu_segcblist_get_seglen(struct rcu_segcblist *rsclp, int seg)
> > >  	return READ_ONCE(rsclp->seglen[seg]);
> > >  }
> > >  
> > > +/* Return number of callbacks in segmented callback list by totalling seglen. */
> > > +long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp)
> > > +{
> > > +	long len = 0;
> > > +	int i;
> > > +
> > > +	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
> > > +		len += rcu_segcblist_get_seglen(rsclp, i);
> > > +
> > > +	return len;
> > > +}
> > > +
> > >  /* Set the length of a segment of the rcu_segcblist structure. */
> > >  static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long v)
> > >  {
> > > diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> > > index cd35c9faaf51..46a42d77f7e1 100644
> > > --- a/kernel/rcu/rcu_segcblist.h
> > > +++ b/kernel/rcu/rcu_segcblist.h
> > > @@ -15,6 +15,9 @@ static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
> > >  	return READ_ONCE(rclp->len);
> > >  }
> > >  
> > > +/* Return number of callbacks in segmented callback list by totalling seglen. */
> > > +long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp);
> > > +
> > >  void rcu_cblist_init(struct rcu_cblist *rclp);
> > >  void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
> > >  void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index f5b61e10f1de..91e35b521e51 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2553,6 +2553,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> > >  	WARN_ON_ONCE(count == 0 && !rcu_segcblist_empty(&rdp->cblist));
> > >  	WARN_ON_ONCE(!IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> > >  		     count != 0 && rcu_segcblist_empty(&rdp->cblist));
> > > +	WARN_ON_ONCE(count == 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) != 0);
> > > +	WARN_ON_ONCE(count != 0 && rcu_segcblist_n_segment_cbs(&rdp->cblist) == 0);
> > >  
> > >  	rcu_nocb_unlock_irqrestore(rdp, flags);
> > >  
> > > -- 
> > > 2.29.2.299.gdc1121823c-goog