Re: [PATCH nf] netfilter: nf_tables: unconditionally flush pending work before notifier

[Hillf Danton <hdanton@sina.com>]

On Sun, 7 Jul 2024 10:08:24 +0200 Florian Westphal <fw@strlen.de>
> Hillf Danton <hdanton@sina.com> wrote:
> > > I think this change might be useful as it also documents
> > > this requirement.
> > 
> > Yes it is boy and the current reproducer triggered another warning [1,2].
> > 
> > [1] https://lore.kernel.org/lkml/20240706231332.3261-1-hdanton@sina.com/
> 
> The WARN is incorrect.  The destroy list can be non-empty; i already
> tried to explain why.
>
That warning as-is could be false positive but it could be triggered with a
single netns.

	cpu1		cpu2		cpu3
	---		---		---
					nf_tables_trans_destroy_work()
					spin_lock(&nf_tables_destroy_list_lock);

					// 1) clear the destroy list
					list_splice_init(&nf_tables_destroy_list, &head);
					spin_unlock(&nf_tables_destroy_list_lock);

			nf_tables_commit_release()
			spin_lock(&nf_tables_destroy_list_lock);

			// 2) refill the destroy list
			list_splice_tail_init(&nft_net->commit_list, &nf_tables_destroy_list);
			spin_unlock(&nf_tables_destroy_list_lock);
			schedule_work(&trans_destroy_work);
			mutex_unlock(&nft_net->commit_mutex);

	nft_rcv_nl_event()
	mutex_lock(&nft_net->commit_mutex);
	flush_work(&trans_destroy_work);
	  start_flush_work()
	    insert_wq_barrier()
	    /*
	     * If @target is currently being executed, schedule the
	     * barrier to the worker; otherwise, put it after @target.
	     */

	// 3) flush work ends with the refilled destroy list left intact
	tear tables down