Re: [bpf PATCH 2/2] bpf: xdp, remove no longer required rcu_read_{un}lock()

[Toshiaki Makita <toshiaki.makita1@gmail.com>]

On 2020/01/09 6:35, John Fastabend wrote:
> Now that we depend on rcu_call() and synchronize_rcu() to also wait
> for preempt_disabled region to complete the rcu read critical section
> in __dev_map_flush() is no longer relevant.
> 
> These originally ensured the map reference was safe while a map was
> also being free'd. But flush by new rules can only be called from
> preempt-disabled NAPI context. The synchronize_rcu from the map free
> path and the rcu_call from the delete path will ensure the reference
> here is safe. So lets remove the rcu_read_lock and rcu_read_unlock
> pair to avoid any confusion around how this is being protected.
> 
> If the rcu_read_lock was required it would mean errors in the above
> logic and the original patch would also be wrong.
> 
> Fixes: 0536b85239b84 ("xdp: Simplify devmap cleanup")
> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> ---
>   kernel/bpf/devmap.c |    2 --
>   1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index f0bf525..0129d4a 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -378,10 +378,8 @@ void __dev_map_flush(void)
>   	struct list_head *flush_list = this_cpu_ptr(&dev_map_flush_list);
>   	struct xdp_bulk_queue *bq, *tmp;
>   
> -	rcu_read_lock();
>   	list_for_each_entry_safe(bq, tmp, flush_list, flush_node)
>   		bq_xmit_all(bq, XDP_XMIT_FLUSH);
> -	rcu_read_unlock();

I introduced this lock because some drivers have assumption that
.ndo_xdp_xmit() is called under RCU. (commit 86723c864063)

Maybe devmap deletion logic does not need this anymore, but is it
OK to drivers?

Toshiaki Makita