The shrinker resets the lazy callbacks counter in order to trigger the
pending lazy queue flush though the rcuog kthread. The counter reset is
protected by the ->nocb_lock against concurrent accesses...except
for one of them. Here is a list of existing synchronized readers/writer:

1) The first lazy enqueuer (incrementing ->lazy_len to 1) does so under
   ->nocb_lock and ->nocb_bypass_lock.

2) The further lazy enqueuers (incrementing ->lazy_len above 1) do so
   under ->nocb_bypass_lock _only_.

3) The lazy flush checks and resets to 0 under ->nocb_lock and
	->nocb_bypass_lock.

The shrinker protects its ->lazy_len reset against cases 1) and 3) but
not against 2). As such, setting ->lazy_len to 0 under the ->nocb_lock
may be cancelled right away by an overwrite from an enqueuer, leading
rcuog to ignore the flush.

To avoid that, use the proper bypass flush API which takes care of all
those details.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/rcu/tree_nocb.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 1a86883902ce..c321fce2af8e 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1364,7 +1364,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 			continue;
 
 		rcu_nocb_lock_irqsave(rdp, flags);
-		WRITE_ONCE(rdp->lazy_len, 0);
+		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
 		rcu_nocb_unlock_irqrestore(rdp, flags);
 		wake_nocb_gp(rdp, false);
 		sc->nr_to_scan -= _count;
-- 
2.34.1

On Wed, Mar 29, 2023 at 06:02:01PM +0200, Frederic Weisbecker wrote:
> The shrinker resets the lazy callbacks counter in order to trigger the
> pending lazy queue flush though the rcuog kthread. The counter reset is
> protected by the ->nocb_lock against concurrent accesses...except
> for one of them. Here is a list of existing synchronized readers/writer:
> 
> 1) The first lazy enqueuer (incrementing ->lazy_len to 1) does so under
>    ->nocb_lock and ->nocb_bypass_lock.
> 
> 2) The further lazy enqueuers (incrementing ->lazy_len above 1) do so
>    under ->nocb_bypass_lock _only_.
> 
> 3) The lazy flush checks and resets to 0 under ->nocb_lock and
> 	->nocb_bypass_lock.
> 
> The shrinker protects its ->lazy_len reset against cases 1) and 3) but
> not against 2). As such, setting ->lazy_len to 0 under the ->nocb_lock
> may be cancelled right away by an overwrite from an enqueuer, leading
> rcuog to ignore the flush.
> 
> To avoid that, use the proper bypass flush API which takes care of all
> those details.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/rcu/tree_nocb.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 1a86883902ce..c321fce2af8e 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1364,7 +1364,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  			continue;
>  
>  		rcu_nocb_lock_irqsave(rdp, flags);
> -		WRITE_ONCE(rdp->lazy_len, 0);
> +		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));

And I do feel much better about this version.  ;-)

>  		rcu_nocb_unlock_irqrestore(rdp, flags);
>  		wake_nocb_gp(rdp, false);
>  		sc->nr_to_scan -= _count;
> -- 
> 2.34.1
>