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

[Joel Fernandes]

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"));

Oh... so the bug is this? Good catch...


CPU 0                                           CPU 1

                                                rcu_gp_init()
                                                      rcu_seq_start(rcu_state.gp_seq)
sychronize_rcu_normal()
      rs.head.func
      	= (void *) get_state_synchronize_rcu();
                   // save rcu_state.gp_seq
      rcu_sr_normal_add_req() ->
            llist_add(rcu_state.srs_next)
      (void) start_poll_synchronize_rcu();


                                                      sr_normal_gp_init()
                                                            llist_add(wait_head, &rcu_state.srs_next);
							    // pick up the
							    // injected WH
                                                            rcu_state.srs_wait_tail = wait_head;

                                                rcu_gp_cleanup()
                                                      rcu_seq_end(&rcu_state.gp_seq);
                                                      sr_normal_complete()
                                                            WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
                                                            !poll_state_synchronize_rcu(oldstate),

Where as reordering sr_normal_gp_init() prevents this:

                                                rcu_gp_init()

                                                      sr_normal_gp_init()
							    // WH has not
							    // been injected
							    // so nothing to
							    // wait on

                                                      rcu_seq_start(rcu_state.gp_seq)
sychronize_rcu_normal()
      rs.head.func
      	= (void *) get_state_synchronize_rcu();
                   // save rcu_state.gp_seq
      rcu_sr_normal_add_req() ->
            llist_add(rcu_state.srs_next)
      (void) start_poll_synchronize_rcu();

                                                rcu_gp_cleanup()
                                                      rcu_seq_end(&rcu_state.gp_seq);
                                                      // sr_normal_complete()
						      // wont do anything so
						      // no warning

Did I get that right?

I think this is a real bug AFAICS, hoping all the memory barriers are in
place to make sure the code reordering also correctly orders the accesses.
I'll double check that.

I also feel its 'theoretical', because as long as rcu_gp_init() and
rcu_gp_cleanup() are properly ordered WRT pre-existing readers, then
synchronize_rcu_normal() still waits for pre-existing readers even though its
a bit confused about the value of the cookies.

For the fix,
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

(If possible, include a Link: to my (this) post so that the sequence of
events is further clarified.)

thanks,

 - Joel