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

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Date:   Sun, 8 Mar 2020 13:42:15 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paul Blakey <paulb@...lanox.com>,
        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 3/8/20 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);
        }
}

> 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,
>>>