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
