netdev - Re: [PATCH 12/14] net: sched: retry action check-insert on concurrent modification

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Date:   Wed, 16 May 2018 14:26:00 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Vlad Buslov <vladbu@...lanox.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        xiyou.wangcong@...il.com, pablo@...filter.org,
        kadlec@...ckhole.kfki.hu, fw@...len.de, ast@...nel.org,
        daniel@...earbox.net, edumazet@...gle.com, keescook@...omium.org,
        linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
        coreteam@...filter.org, kliteyn@...lanox.com
Subject: Re: [PATCH 12/14] net: sched: retry action check-insert on
 concurrent modification

Wed, May 16, 2018 at 01:55:06PM CEST, vladbu@...lanox.com wrote:
>
>On Wed 16 May 2018 at 09:59, Jiri Pirko <jiri@...nulli.us> wrote:
>> Mon, May 14, 2018 at 04:27:13PM CEST, vladbu@...lanox.com wrote:
>>>Retry check-insert sequence in action init functions if action with same
>>>index was inserted concurrently.
>>>
>>>Signed-off-by: Vlad Buslov <vladbu@...lanox.com>
>>>---
>>> net/sched/act_bpf.c        | 8 +++++++-
>>> net/sched/act_connmark.c   | 8 +++++++-
>>> net/sched/act_csum.c       | 8 +++++++-
>>> net/sched/act_gact.c       | 8 +++++++-
>>> net/sched/act_ife.c        | 8 +++++++-
>>> net/sched/act_ipt.c        | 8 +++++++-
>>> net/sched/act_mirred.c     | 8 +++++++-
>>> net/sched/act_nat.c        | 8 +++++++-
>>> net/sched/act_pedit.c      | 8 +++++++-
>>> net/sched/act_police.c     | 9 ++++++++-
>>> net/sched/act_sample.c     | 8 +++++++-
>>> net/sched/act_simple.c     | 9 ++++++++-
>>> net/sched/act_skbedit.c    | 8 +++++++-
>>> net/sched/act_skbmod.c     | 8 +++++++-
>>> net/sched/act_tunnel_key.c | 9 ++++++++-
>>> net/sched/act_vlan.c       | 9 ++++++++-
>>> 16 files changed, 116 insertions(+), 16 deletions(-)
>>>
>>>diff --git a/net/sched/act_bpf.c b/net/sched/act_bpf.c
>>>index 5554bf7..7e20fdc 100644
>>>--- a/net/sched/act_bpf.c
>>>+++ b/net/sched/act_bpf.c
>>>@@ -299,10 +299,16 @@ static int tcf_bpf_init(struct net *net, struct nlattr *nla,
>>> 
>>> 	parm = nla_data(tb[TCA_ACT_BPF_PARMS]);
>>> 
>>>+replay:
>>> 	if (!tcf_idr_check(tn, parm->index, act, bind)) {
>>> 		ret = tcf_idr_create(tn, parm->index, est, act,
>>> 				     &act_bpf_ops, bind, true);
>>>-		if (ret < 0)
>>>+		/* Action with specified index was created concurrently.
>>>+		 * Check again.
>>>+		 */
>>>+		if (parm->index && ret == -ENOSPC)
>>>+			goto replay;
>>>+		else if (ret)
>>
>> Hmm, looks like you are doing the same/very similar thing in every act
>> code. I think it would make sense to introduce a helper function for
>> this purpose.
>
>This code uses goto so it can't be easily refactored into standalone
>function. Could you specify which part of this code you suggest to
>extract?

Hmm, looking at the code, I think that what would help is to have a
helper that would atomically check if index exists and if not, it would
allocate one. Something like:


int tcf_idr_check_alloc(struct tc_action_net *tn, u32 *index,
			struct tc_action **a, int bind)
{
	struct tcf_idrinfo *idrinfo = tn->idrinfo;
	struct tc_action *p;
	int err;

	spin_lock(&idrinfo->lock);
	if (*index) {
		p = idr_find(&idrinfo->action_idr, *index);
		if (p) {
			if (bind)
	   			p->tcfa_bindcnt++;
			p->tcfa_refcnt++;
			*a = p;
			err = 0;
		} else {
			*a = NULL;
			err = idr_alloc_u32(idr, NULL, index,
					    *index, GFP_ATOMIC);
		}
	} else {
		*index = 1;
		*a = NULL;
		err = idr_alloc_u32(idr, NULL, index, UINT_MAX, GFP_ATOMIC);
	}
	spin_unlock(&idrinfo->lock);
	return err;
}

The act code would just check if "a" is NULL and if so, it would call
tcf_idr_create() with allocated index as arg.


>
>>
>> [...]
>