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

Paul E. McKenney
2023-03-22T23:18:32Z

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
>