Re: [PATCH 01/11] rcu: avoid leaking exp_deferred_qs into next GP

Paul E. McKenney
2019-10-31T13:43:54Z

On Thu, Oct 31, 2019 at 10:07:56AM +0000, Lai Jiangshan wrote:
> If exp_deferred_qs is incorrectly set and leaked to the next
> exp GP, it may cause the next GP to be incorrectly prematurely
> completed.

Could you please provide the sequence of events leading to a such a
failure?

Also, did you provoke such a failure in testing? If so, an upgrade
to rcutorture would be good, so please tell me what you did to make
the failure happen.

I do like the reduction in state space, but I am a bit concerned about
the potential increase in contention on rnp->lock. Thoughts?

Thanx, Paul

> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> kernel/rcu/tree_exp.h | 23 ++++++++++++++---------
> 1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index a0e1e51c51c2..6dec21909b30 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -603,6 +603,18 @@ static void rcu_exp_handler(void *unused)
> struct rcu_node *rnp = rdp->mynode;
> struct task_struct *t = current;
>
> + /*
> + * Note that there is a large group of race conditions that
> + * can have caused this quiescent state to already have been
> + * reported, so we really do need to check ->expmask first.
> + */
> + raw_spin_lock_irqsave_rcu_node(rnp, flags);
> + if (!(rnp->expmask & rdp->grpmask)) {
> + raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> + return;
> + }
> + raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> +
> /*
> * First, the common case of not being in an RCU read-side
> * critical section. If also enabled or idle, immediately
> @@ -628,17 +640,10 @@ static void rcu_exp_handler(void *unused)
> * a future context switch. Either way, if the expedited
> * grace period is still waiting on this CPU, set ->deferred_qs
> * so that the eventual quiescent state will be reported.
> - * Note that there is a large group of race conditions that
> - * can have caused this quiescent state to already have been
> - * reported, so we really do need to check ->expmask.
> */
> if (t->rcu_read_lock_nesting > 0) {
> - raw_spin_lock_irqsave_rcu_node(rnp, flags);
> - if (rnp->expmask & rdp->grpmask) {
> - rdp->exp_deferred_qs = true;
> - t->rcu_read_unlock_special.b.exp_hint = true;
> - }
> - raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> + rdp->exp_deferred_qs = true;
> + WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> return;
> }
>
> --
> 2.20.1
>