Re: [PATCH 1/3] rcu/exp: Protect against early QS report

Paul E. McKenney
2025-02-14T09:10:48Z

On Fri, Feb 14, 2025 at 12:25:57AM +0100, Frederic Weisbecker wrote:
> When a grace period is started, the ->expmask of each node is set up
> from sync_exp_reset_tree(). Then later on each leaf node also initialize
> its ->exp_tasks pointer.
>
> This means that the initialization of the quiescent state of a node and
> the initialization of its blocking tasks happen with an unlocked node
> gap in-between.
>
> It happens to be fine because nothing is expected to report an exp
> quiescent state within this gap, since no IPI have been issued yet and
> every rdp's ->cpu_no_qs.b.exp should be false.
>
> However if it were to happen by accident, the quiescent state could be
> reported and propagated while ignoring tasks that blocked _before_ the
> start of the grace period.
>
> Prevent such trouble to happen in the future and initialize both the
> quiescent states mask to report and the blocked tasks head from the same
> node locked block.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Thank you for looking into this!

One question: What happens if a CPU has tasks pending during the
call to sync_exp_reset_tree(), but all of these tasks complete
their RCU read-side critical sections before execution reaches
__sync_rcu_exp_select_node_cpus()?

(My guess is that all is well, but even if so, it would be good to record
why in the commit log.)

Thanx, Paul

> ---
> kernel/rcu/tree_exp.h | 14 +++++++-------
> 1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 8d4895c854c5..caff16e441d1 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -141,6 +141,13 @@ static void __maybe_unused sync_exp_reset_tree(void)
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
> WARN_ON_ONCE(rnp->expmask);
> WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
> + /*
> + * Need to wait for any blocked tasks as well. Note that
> + * additional blocking tasks will also block the expedited GP
> + * until such time as the ->expmask bits are cleared.
> + */
> + if (rcu_is_leaf_node(rnp) && rcu_preempt_has_tasks(rnp))
> + WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next);
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> }
> }
> @@ -393,13 +400,6 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
> }
> mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
>
> - /*
> - * Need to wait for any blocked tasks as well. Note that
> - * additional blocking tasks will also block the expedited GP
> - * until such time as the ->expmask bits are cleared.
> - */
> - if (rcu_preempt_has_tasks(rnp))
> - WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next);
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>
> /* IPI the remaining CPUs for expedited quiescent state. */
> --
> 2.46.0
>