Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Mar 22, 2023 at 08:44:53PM +0100, Frederic Weisbecker wrote:
> The shrinker may run concurrently with callbacks (de-)offloading. As
> such, calling rcu_nocb_lock() is very dangerous because it does a
> conditional locking. The worst outcome is that rcu_nocb_lock() doesn't
> lock but rcu_nocb_unlock() eventually unlocks, or the reverse, creating
> an imbalance.
> 
> Fix this with protecting against (de-)offloading using the barrier mutex.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Good catch!!!  A few questions, comments, and speculations below.

							Thanx, Paul

> ---
>  kernel/rcu/tree_nocb.h | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index f2280616f9d5..dd9b655ae533 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1336,13 +1336,25 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  	unsigned long flags;
>  	unsigned long count = 0;
>  
> +	/*
> +	 * Protect against concurrent (de-)offloading. Otherwise nocb locking
> +	 * may be ignored or imbalanced.
> +	 */
> +	mutex_lock(&rcu_state.barrier_mutex);

I was worried about this possibly leading to out-of-memory deadlock,
but if I recall correctly, the (de-)offloading process never allocates
memory, so this should be OK?

The other concern was that the (de-)offloading operation might take a
long time, but the usual cause for that is huge numbers of callbacks,
in which case letting them free their memory is not necessarily a bad
strategy.

> +
>  	/* Snapshot count of all CPUs */
>  	for_each_possible_cpu(cpu) {
>  		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> -		int _count = READ_ONCE(rdp->lazy_len);
> +		int _count;
> +
> +		if (!rcu_rdp_is_offloaded(rdp))
> +			continue;

If the CPU is offloaded, isn't ->lazy_len guaranteed to be zero?

Or can it contain garbage after a de-offloading operation?

> +		_count = READ_ONCE(rdp->lazy_len);
>  
>  		if (_count == 0)
>  			continue;
> +
>  		rcu_nocb_lock_irqsave(rdp, flags);
>  		WRITE_ONCE(rdp->lazy_len, 0);
>  		rcu_nocb_unlock_irqrestore(rdp, flags);
> @@ -1352,6 +1364,9 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
>  		if (sc->nr_to_scan <= 0)
>  			break;
>  	}
> +
> +	mutex_unlock(&rcu_state.barrier_mutex);
> +
>  	return count ? count : SHRINK_STOP;
>  }
>  
> -- 
> 2.34.1
>