On Sun, Mar 02, 2025 at 07:15:07PM -0500, Joel Fernandes wrote:
> 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) <j...@joelfernandes.org>
Oops, this should be:
Reviewed-by: Joel Fernandes <joelagn...@nvidia.com>
;-)
thanks,
- Joel
>
> (If possible, include a Link: to my (this) post so that the sequence of
> events is further clarified.)
>
> thanks,
>
> - Joel
>
