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

Jiri Pirko
2018-05-16T14:13:23Z

Wed, May 16, 2018 at 03:52:20PM CEST, vladbu@mellanox.com wrote:
>
>On Wed 16 May 2018 at 13:21, Jiri Pirko <jiri@resnulli.us> wrote:
>> 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?
>
>Lets look at mirred for exmple:
> exists = tcf_idr_check(tn, parm->index, a, bind);
> if (exists && bind)
> return 0;
>
> switch (parm->eaction) {
> case TCA_EGRESS_MIRROR:
> case TCA_EGRESS_REDIR:
> case TCA_INGRESS_REDIR:
> case TCA_INGRESS_MIRROR:
> break;
> default:
> if (exists)
> tcf_idr_release(*a, bind);
> NL_SET_ERR_MSG_MOD(extack, "Unknown mirred option");
> return -EINVAL;
> }
> if (parm->ifindex) {
> dev = __dev_get_by_index(net, parm->ifindex);
> if (dev == NULL) {
> if (exists)
> tcf_idr_release(*a, bind);
> return -ENODEV;
> }
> mac_header_xmit = dev_is_mac_header_xmit(dev);
> } else {
> dev = NULL;
> }
>
> if (!exists) {
> if (!dev) {
> NL_SET_ERR_MSG_MOD(extack, "Specified device does not exist");
> return -EINVAL;
> }
> ret = tcf_idr_create(tn, parm->index, est, a,
> &act_mirred_ops, bind, true);
> /* Action with specified index was created concurrently.
> * Check again.
> */
> if (parm->index && ret == -ENOSPC)
> goto replay;
> else if (ret)
> return ret;
>
>There are several returns and cleanup is only performed when action
>exists. So all code like that will have to be audited to also remove
>index from idr, otherwise idr handles leak on return.

Yeah. You have to take care of the error path.

>
>>
>> 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;
>> }
>>
>
>So users of action idr that might perform concurrent lookups are also
>have to be changed to check for error pointers, that now can be inserted
>into idr? Seems like a complex change...

You can just add a simple check into tcf_idr_lookup(). Where else?

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