Re: [PATCH v4 3/3] rcu: Use _full() API to debug synchronize_rcu()

[Uladzislau Rezki <urezki@gmail.com>]

On Fri, Feb 28, 2025 at 05:08:49PM -0800, Paul E. McKenney wrote:
> On Fri, Feb 28, 2025 at 11:59:55AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 28, 2025 at 08:12:51PM +0100, Uladzislau Rezki wrote:
> > > Hello, Paul!
> > > 
> > > > > > > > 
> > > > > > > > Except that I got this from overnight testing of rcu/dev on the shared
> > > > > > > > RCU tree:
> > > > > > > > 
> > > > > > > > WARNING: CPU: 5 PID: 14 at kernel/rcu/tree.c:1636 rcu_sr_normal_complete+0x5c/0x80
> > > > > > > > 
> > > > > > > > I see this only on TREE05.  Which should not be too surprising, given
> > > > > > > > that this is the scenario that tests it.  It happened within five minutes
> > > > > > > > on all 14 of the TREE05 runs.
> > > > > > > > 
> > > > > > > Hm.. This is not fun. I tested this on my system and i did not manage to
> > > > > > > trigger this whereas you do. Something is wrong.
> > > > > > 
> > > > > > If you have a debug patch, I would be happy to give it a go.
> > > > > > 
> > > > > I can trigger it. But.
> > > > > 
> > > > > Some background. I tested those patches during many hours on the stable
> > > > > kernel which is 6.13. On that kernel i was not able to trigger it. Running
> > > > > the rcutorture on the our shared "dev" tree, which i did now, triggers this
> > > > > right away.
> > > > 
> > > > Bisection?  (Hey, you knew that was coming!)
> > > > 
> > > Looks like this: rcu: Fix get_state_synchronize_rcu_full() GP-start detection
> > > 
> > > After revert in the dev, rcutorture passes TREE05, 16 instances.
> > 
> > Huh.  We sure don't get to revert that one...
> > 
> > Do we have a problem with the ordering in rcu_gp_init() between the calls
> > to rcu_seq_start() and portions of rcu_sr_normal_gp_init()?  For example,
> > do we need to capture the relevant portion of the list before the call
> > to rcu_seq_start(), and do the grace-period-start work afterwards?
> 
> I tried moving the call to rcu_sr_normal_gp_init() before the call to
> rcu_seq_start() and got no failures in a one-hour run of 200*TREE05.
> Which does not necessarily mean that this is the correct fix, but I
> figured that it might at least provide food for thought.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 48384fa2eaeb8..d3efeff7740e7 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1819,10 +1819,10 @@ static noinline_for_stack bool rcu_gp_init(void)
>  
>  	/* Advance to a new grace period and initialize state. */
>  	record_gp_stall_check_time();
> +	start_new_poll = rcu_sr_normal_gp_init();
>  	/* Record GP times before starting GP, hence rcu_seq_start(). */
>  	rcu_seq_start(&rcu_state.gp_seq);
>  	ASSERT_EXCLUSIVE_WRITER(rcu_state.gp_seq);
> -	start_new_poll = rcu_sr_normal_gp_init();
>  	trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, TPS("start"));
>  	rcu_poll_gp_seq_start(&rcu_state.gp_seq_polled_snap);
>  	raw_spin_unlock_irq_rcu_node(rnp);
>
Running this 24 hours already. TREE05 * 16 scenario. I do not see any
warnings yet. There is a race, indeed. The gp_seq is moved forward,
wheres clients can still come until rcu_sr_normal_gp_init() places a
dummy-wait-head for this GP.

Thank you for testing Paul and looking to this :)

--
Uladzislau Rezki