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

[Paul E. McKenney]

On Wed, Mar 29, 2023 at 06:02:00PM +0200, 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.
> Although if the barrier mutex is contended, which should be rare, then
> step aside so as not to trigger a mutex VS allocation
> dependency chain.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
>  kernel/rcu/tree_nocb.h | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index f2280616f9d5..1a86883902ce 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1336,13 +1336,33 @@ 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.
> +	 */
> +	if (!mutex_trylock(&rcu_state.barrier_mutex)) {

This looks much better, thank you!

> +		/*
> +		 * But really don't insist if barrier_mutex is contended since we
> +		 * can't guarantee that it will never engage in a dependency
> +		 * chain involving memory allocation. The lock is seldom contended
> +		 * anyway.
> +		 */
> +		return 0;
> +	}
> +
>  	/* 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;
> +
> +		_count = READ_ONCE(rdp->lazy_len);
>  
>  		if (_count == 0)
>  			continue;
> +

And I just might have unconfused myself here.  We get here only if this
CPU is offloaded, in which case it might also have non-zero ->lazy_len,
so this is in fact *not* dead code.

>  		rcu_nocb_lock_irqsave(rdp, flags);
>  		WRITE_ONCE(rdp->lazy_len, 0);
>  		rcu_nocb_unlock_irqrestore(rdp, flags);
> @@ -1352,6 +1372,9 @@ 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
>