netdev - Re: [Patch net-next] net_sched: fix an extack message in tcf_block

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Date:   Thu, 27 Sep 2018 14:16:14 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>, netdev@...r.kernel.org
Cc:     jiri@...nulli.us, jhs@...atatu.com, vladbu@...lanox.com
Subject: Re: [Patch net-next] net_sched: fix an extack message in
 tcf_block_find()

On 09/27/2018 01:42 PM, Cong Wang wrote:
> It is clearly a copy-n-paste.
> 
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/sched/cls_api.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> index 3de47e99b788..8dd7f8af6d54 100644
> --- a/net/sched/cls_api.c
> +++ b/net/sched/cls_api.c
> @@ -655,7 +655,7 @@ static struct tcf_block *tcf_block_find(struct net *net, struct Qdisc **q,
>  
>  		*q = qdisc_refcount_inc_nz(*q);
>  		if (!*q) {
> -			NL_SET_ERR_MSG(extack, "Parent Qdisc doesn't exists");
> +			NL_SET_ERR_MSG(extack, "Can't increase Qdisc refcount");

I am not sure it was a copy-n-paste.

Qdisc refcount business is kernel internal.
If we can not increase the refcount, this is precisely because this qdisc is about
to be destroyed. Nothing fundamentally different than having this thread delayed a bit
and qdisc_lookup_rcu() returning NULL in the first place.

This also means that using RCU for control path is problematic, as surely the caller
of this interface would prefer something that succeeds, even if this means
waiting a bit in the kernel.

Or are we willing to change ip command and make it restart failed syscalls ?