netdev - Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu() similar to kfree

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Date: Sun, 25 Jun 2023 19:15:27 +0800
From: Hou Tao <houtao@...weicloud.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>, daniel@...earbox.net,
 andrii@...nel.org, void@...ifault.com, paulmck@...nel.org
Cc: tj@...nel.org, rcu@...r.kernel.org, netdev@...r.kernel.org,
 bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v2 bpf-next 12/13] bpf: Introduce bpf_mem_free_rcu()
 similar to kfree_rcu().

Hi,

On 6/24/2023 11:13 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
>
> Introduce bpf_mem_[cache_]free_rcu() similar to kfree_rcu().
> Unlike bpf_mem_[cache_]free() that links objects for immediate reuse into
> per-cpu free list the _rcu() flavor waits for RCU grace period and then moves
> objects into free_by_rcu_ttrace list where they are waiting for RCU
> task trace grace period to be freed into slab.
SNIP
>  
>  static void free_mem_alloc_no_barrier(struct bpf_mem_alloc *ma)
> @@ -498,8 +566,8 @@ static void free_mem_alloc_no_barrier(struct bpf_mem_alloc *ma)
>  
>  static void free_mem_alloc(struct bpf_mem_alloc *ma)
>  {
> -	/* waiting_for_gp_ttrace lists was drained, but __free_rcu might
> -	 * still execute. Wait for it now before we freeing percpu caches.
> +	/* waiting_for_gp[_ttrace] lists were drained, but RCU callbacks
> +	 * might still execute. Wait for them.
>  	 *
>  	 * rcu_barrier_tasks_trace() doesn't imply synchronize_rcu_tasks_trace(),
>  	 * but rcu_barrier_tasks_trace() and rcu_barrier() below are only used
I think an extra rcu_barrier() before rcu_barrier_tasks_trace() is still
needed here, otherwise free_mem_alloc will not wait for inflight
__free_by_rcu() and there will oops in rcu_do_batch().