netdev - Re: [PATCH net-next v6 1/3] net/sched: act

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Date:   Mon, 9 Mar 2020 10:02:07 +0200
From:   Paul Blakey <paulb@...lanox.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Oz Shlomo <ozsh@...lanox.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Vlad Buslov <vladbu@...lanox.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...lanox.com>, Roi Dayan <roid@...lanox.com>
Subject: Re: [PATCH net-next v6 1/3] net/sched: act_ct: Create nf flow table
 per zone


On 3/8/2020 10:42 PM, Eric Dumazet wrote:
>
> On 3/8/20 12:15 AM, Paul Blakey wrote:
>> On 3/8/2020 10:11 AM, Paul Blakey wrote:
>>
>>> iirc I did the spin lock bh because we can be called from queue work rcu handler , so I wanted to disable soft irq.
>>>
>>> I got a possible deadlock splat for that.
>> Here I meant this call rcu:
>>
>> static void tcf_ct_cleanup(struct tc_action *a)
>> {
>>> -------struct tcf_ct_params *params;
>>> -------struct tcf_ct *c = to_ct(a);
>>> -------params = rcu_dereference_protected(c->params, 1);
>>> -------if (params)
>>> ------->-------call_rcu(&params->rcu, tcf_ct_params_free);
>> }
>>
> Yes, understood, but to solve this problem we had many other choices,
> and still keeping GFP_KERNEL allocations and a mutex for control path.
>
> Have you read my patch ?
>
> By not even trying to get a spinlock in tcf_ct_flow_table_put(),
> and instead use a refcount for ->ref, we avoid having this issue in the first place.
>
> static void tcf_ct_flow_table_put(struct tcf_ct_params *params)
> {
>         struct tcf_ct_flow_table *ct_ft = params->ct_ft;
>
>         if (refcount_dec_and_test(&params->ct_ft->ref)) {
>                 rhashtable_remove_fast(&zones_ht, &ct_ft->node, zones_params);
>                 INIT_RCU_WORK(&ct_ft->rwork, tcf_ct_flow_table_cleanup_work);
>                 queue_rcu_work(act_ct_wq, &ct_ft->rwork);
>         }
> }
Sorry missed that, thanks for the fix.
>> static void tcf_ct_params_free(struct rcu_head *head)
>> {
>>> -------struct tcf_ct_params *params = container_of(head,
>>> ------->------->------->------->------->-------    struct tcf_ct_params, rcu);
>>> -------tcf_ct_flow_table_put(params);
>> ...
>>
>>
>>>
>>> On 3/7/2020 10:53 PM, Eric Dumazet wrote:
>>>
>>>> On 3/7/20 12:12 PM, Eric Dumazet wrote:
>>>>> On 3/3/20 7:57 AM, Paul Blakey wrote:
>>>>>> Use the NF flow tables infrastructure for CT offload.
>>>>>>
>>>>>> Create a nf flow table per zone.
>>>>>>
>>>>>> Next patches will add FT entries to this table, and do
>>>>>> the software offload.
>>>>>>
>>>>>> Signed-off-by: Paul Blakey <paulb@...lanox.com>
>>>>>> Reviewed-by: Jiri Pirko <jiri@...lanox.com>
>>>>>> ---
>>>>>>   v4->v5:
>>>>>>     Added reviewed by Jiri, thanks!
>>>>>>   v3->v4:
>>>>>>     Alloc GFP_ATOMIC
>>>>>>   v2->v3:
>>>>>>     Ditch re-locking to alloc, and use atomic allocation
>>>>>>   v1->v2:
>>>>>>     Use spin_lock_bh instead of spin_lock, and unlock for alloc (as it can sleep)
>>>>>>     Free ft on last tc act instance instead of last instance + last offloaded tuple,
>>>>>>     this removes cleanup cb and netfilter patches, and is simpler
>>>>>>     Removed accidental mlx5/core/en_tc.c change
>>>>>>     Removed reviewed by Jiri - patch changed
>>>>>>
>>>>>> +	err = nf_flow_table_init(&ct_ft->nf_ft);
>>>>> This call is going to allocate a rhashtable (GFP_KERNEL allocations that might sleep)
>>>>>
>>>>> Since you still hold zones_lock spinlock, a splat should occur.
>>>>>
>>>>> "BUG: sleeping function called from invalid context in  ..."
>>>>>
>>>>> DEBUG_ATOMIC_SLEEP=y is your friend.
>>>>>
>>>>> And it is always a good thing to make sure a patch does not trigger a lockdep splat
>>>>>
>>>>> CONFIG_PROVE_LOCKING=y
>>>> Also abusing a spinlock and GFP_ATOMIC allocations in control path is highly discouraged.
>>>>
>>>> I can not test the following fix, any objections before I submit this officially ?
>>>>
>>>> diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
>>>> index 23eba61f0f819212a3522c3c63b938d0b8d997e2..3d9e678d7d5336f1746035745b091bea0dcb5fdd 100644
>>>> --- a/net/sched/act_ct.c
>>>> +++ b/net/sched/act_ct.c
>>>> @@ -35,15 +35,15 @@
>>>>  
>>>>  static struct workqueue_struct *act_ct_wq;
>>>>  static struct rhashtable zones_ht;
>>>> -static DEFINE_SPINLOCK(zones_lock);
>>>> +static DEFINE_MUTEX(zones_mutex);
>>>>  
>>>>  struct tcf_ct_flow_table {
>>>>         struct rhash_head node; /* In zones tables */
>>>>  
>>>>         struct rcu_work rwork;
>>>>         struct nf_flowtable nf_ft;
>>>> +       refcount_t ref;
>>>>         u16 zone;
>>>> -       u32 ref;
>>>>  
>>>>         bool dying;
>>>>  };
>>>> @@ -64,14 +64,15 @@ static int tcf_ct_flow_table_get(struct tcf_ct_params *params)
>>>>         struct tcf_ct_flow_table *ct_ft;
>>>>         int err = -ENOMEM;
>>>>  
>>>> -       spin_lock_bh(&zones_lock);
>>>> +       mutex_lock(&zones_mutex);
>>>>         ct_ft = rhashtable_lookup_fast(&zones_ht, &params->zone, zones_params);
>>>> -       if (ct_ft)
>>>> -               goto take_ref;
>>>> +       if (ct_ft && refcount_inc_not_zero(&ct_ft->ref))
>>>> +               goto out_unlock;
>>>>  
>>>> -       ct_ft = kzalloc(sizeof(*ct_ft), GFP_ATOMIC);
>>>> +       ct_ft = kzalloc(sizeof(*ct_ft), GFP_KERNEL);
>>>>         if (!ct_ft)
>>>>                 goto err_alloc;
>>>> +       refcount_set(&ct_ft->ref, 1);
>>>>  
>>>>         ct_ft->zone = params->zone;
>>>>         err = rhashtable_insert_fast(&zones_ht, &ct_ft->node, zones_params);
>>>> @@ -84,10 +85,9 @@ static int tcf_ct_flow_table_get(struct tcf_ct_params *params)
>>>>                 goto err_init;
>>>>  
>>>>         __module_get(THIS_MODULE);
>>>> -take_ref:
>>>> +out_unlock:
>>>>         params->ct_ft = ct_ft;
>>>> -       ct_ft->ref++;
>>>> -       spin_unlock_bh(&zones_lock);
>>>> +       mutex_unlock(&zones_mutex);
>>>>  
>>>>         return 0;
>>>>  
>>>> @@ -96,7 +96,7 @@ static int tcf_ct_flow_table_get(struct tcf_ct_params *params)
>>>>  err_insert:
>>>>         kfree(ct_ft);
>>>>  err_alloc:
>>>> -       spin_unlock_bh(&zones_lock);
>>>> +       mutex_unlock(&zones_mutex);
>>>>         return err;
>>>>  }
>>>>  
>>>> @@ -116,13 +116,11 @@ static void tcf_ct_flow_table_put(struct tcf_ct_params *params)
>>>>  {
>>>>         struct tcf_ct_flow_table *ct_ft = params->ct_ft;
>>>>  
>>>> -       spin_lock_bh(&zones_lock);
>>>> -       if (--params->ct_ft->ref == 0) {
>>>> +       if (refcount_dec_and_test(&params->ct_ft->ref)) {
>>>>                 rhashtable_remove_fast(&zones_ht, &ct_ft->node, zones_params);
>>>>                 INIT_RCU_WORK(&ct_ft->rwork, tcf_ct_flow_table_cleanup_work);
>>>>                 queue_rcu_work(act_ct_wq, &ct_ft->rwork);
>>>>         }
>>>> -       spin_unlock_bh(&zones_lock);
>>>>  }
>>>>  
>>>>  static void tcf_ct_flow_table_add(struct tcf_ct_flow_table *ct_ft,
>>>>