Re: [PATCH bpf-next v2 02/15] bpf: Make struct_ops tasks_rcu grace period optional

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2026-06-23 at 10:49 -0700, Amery Hung wrote:
> From: Martin KaFai Lau <martin.lau@kernel.org>
> 
> bpf_struct_ops_map_free() currently waits for both a regular RCU grace
> period and a tasks RCU grace period for every struct_ops map through
> synchronize_rcu_mult(call_rcu, call_rcu_tasks).
> 
> A regular RCU grace period is still required for all struct_ops maps
> because the struct_ops trampoline ksyms requires a rcu grace period
> (take a look at the list_del_rcu in __bpf_ksym_del).
> Add a map_free_pre_rcu() callback so the struct_ops map can remove
> ksyms before bpf_map_put() wait for the regular rcu grace period.
> 
> The tasks RCU grace period is only needed by tcp_congestion_ops.
> Add free_after_tasks_rcu_gp only to struct bpf_struct_ops instead
> of the bpf_map.
> 
> When CONFIG_TASKS_RCU=n, synchronize_rcu_tasks() is the same as
> synchronize_rcu(). Since all struct_ops maps now complete a regular RCU
> grace period before bpf_struct_ops_map_free() runs, skip the extra
> synchronize_rcu_tasks() call in this case.
> 
> This cleanup prepares for a later patch that needs to support
> free_after_mult_rcu_gp.
> 
> Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
> Signed-off-by: Amery Hung <ameryhung@gmail.com>
> ---

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

> @@ -997,24 +1006,8 @@ static void bpf_struct_ops_map_free(struct bpf_map *map)
>  
>  	bpf_struct_ops_map_dissoc_progs(st_map);
>  
> -	bpf_struct_ops_map_del_ksyms(st_map);
> -
> -	/* The struct_ops's function may switch to another struct_ops.
> -	 *
> -	 * For example, bpf_tcp_cc_x->init() may switch to
> -	 * another tcp_cc_y by calling
> -	 * setsockopt(TCP_CONGESTION, "tcp_cc_y").
> -	 * During the switch,  bpf_struct_ops_put(tcp_cc_x) is called
> -	 * and its refcount may reach 0 which then free its
> -	 * trampoline image while tcp_cc_x is still running.
> -	 *
> -	 * A vanilla rcu gp is to wait for all bpf-tcp-cc prog
> -	 * to finish. bpf-tcp-cc prog is non sleepable.
> -	 * A rcu_tasks gp is to wait for the last few insn
> -	 * in the tramopline image to finish before releasing
> -	 * the trampoline image.
> -	 */
> -	synchronize_rcu_mult(call_rcu, call_rcu_tasks);
> +	if (tasks_rcu && IS_ENABLED(CONFIG_TASKS_RCU))
> +		synchronize_rcu_tasks();

As far as I understand, this removes the synchronize_rcu_tasks()
for qdisk, sched_ext, smc and hid struct ops. As far as I can tell,
each one of them employs separate means to guarantee that there won't
be any pending BPF trampolines referring to the image being freed here.
So, the change appears to be safe.

>  
>  	__bpf_struct_ops_map_free(map);
>  }

[...]