netdev - Re: [PATCH net-next] net: sched: flower: refactor reoffload for concurrent access

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Message-ID: <20190422140227.421a6e94@cakuba.netronome.com>
Date:   Mon, 22 Apr 2019 14:02:27 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     netdev@...r.kernel.org, jhs@...atatu.com, xiyou.wangcong@...il.com,
        jiri@...nulli.us, davem@...emloft.net
Subject: Re: [PATCH net-next] net: sched: flower: refactor reoffload for
 concurrent access

On Mon, 22 Apr 2019 10:21:34 +0300, Vlad Buslov wrote:
> Recent changes that introduced unlocked flower did not properly account for
> case when reoffload is initiated concurrently with filter updates. To fix
> the issue, extend flower with 'hw_filters' list that is used to store
> filters that don't have 'skip_hw' flag set. Filter is added to the list
> when it is inserted to hardware and only removed from it after being
> unoffloaded from all drivers that parent block is attached to. This ensures
> that concurrent reoffload can still access filter that is being deleted and
> prevents race condition when driver callback can be removed when filter is
> no longer accessible trough idr, but is still present in hardware.
> 
> Refactor fl_change() to respect new filter reference counter and to release
> filter reference with __fl_put() in case of error, instead of directly
> deallocating filter memory. This allows for concurrent access to filter
> from fl_reoffload() and protects it with reference counting. Refactor
> fl_reoffload() to iterate over hw_filters list instead of idr. Implement
> fl_get_next_hw_filter() helper function that is used to iterate over
> hw_filters list with reference counting and skips filters that are being
> concurrently deleted.
> 
> Fixes: 92149190067d ("net: sched: flower: set unlocked flag for flower proto ops")
> Signed-off-by: Vlad Buslov <vladbu@...lanox.com>
> Reported-by: Jakub Kicinski <jakub.kicinski@...ronome.com>

Perhaps it'd be good to add an ASSERT_RTNL() (maybe with a comment?) 
to fl_reoffload()?

> @@ -382,6 +395,8 @@ static void fl_hw_destroy_filter(struct tcf_proto *tp, struct cls_fl_filter *f,
>  
>  	tc_setup_cb_call(block, TC_SETUP_CLSFLOWER, &cls_flower, false);
>  	spin_lock(&tp->lock);
> +	if (!list_empty(&f->hw_list))
> +		list_del_init(&f->hw_list);

Mm. I thought list_del_init() on an empty list should be fine?

>  	tcf_block_offload_dec(block, &f->flags);
>  	spin_unlock(&tp->lock);
>