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

Jiri Pirko
2018-05-16T13:21:39Z

Wed, May 16, 2018 at 02:43:58PM CEST, vladbu@mellanox.com wrote:
>
>On Wed 16 May 2018 at 12:26, Jiri Pirko <jiri@resnulli.us> wrote:
>> Wed, May 16, 2018 at 01:55:06PM CEST, vladbu@mellanox.com wrote:
>>>
>>>On Wed 16 May 2018 at 09:59, Jiri Pirko <jiri@resnulli.us> wrote:
>>>> Mon, May 14, 2018 at 04:27:13PM CEST, vladbu@mellanox.com wrote:
>>>>>Retry check-insert sequence in action init functions if action with same
>>>>>index was inserted concurrently.
>>>>>
>>>>>Signed-off-by: Vlad Buslov <vladbu@mellanox.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.
>
>What about multiple actions that have arbitrary code between initial
>check and idr allocation that is currently inside tcf_idr_create()?

Why it would be a problem to have them after the allocation?

There is one issue though with my draft. tcf_idr_insert() function
which actually assigns a "p" pointer to the idr index is called later on.
Until that happens, the idr_find() would return NULL even if the index
is actually allocated. We cannot assign "p" in tcf_idr_check_alloc()
because it is allocated only later on in tcf_idr_create(). But that is
resolvable by the following trick:

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;

again:
spin_lock(&idrinfo->lock);
if (*index) {
p = idr_find(&idrinfo->action_idr, *index);
if (IS_ERR(p)) {
/* This means that another process allocated
* index but did not assign the pointer yet.
*/
spin_unlock(&idrinfo->lock);
goto again;
}
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);
idr_replace(&idrinfo->action_idr,
ERR_PTR(-EBUSY), *index);
}
} else {
*index = 1;
*a = NULL;
err = idr_alloc_u32(idr, NULL, index, UINT_MAX, GFP_ATOMIC);
idr_replace(&idrinfo->action_idr, ERR_PTR(-EBUSY), *index);
}
spin_unlock(&idrinfo->lock);
return err;
}

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