netdev - Re: [PATCH v2 net-next 06/12] qed: Add LL2 slowpath handling

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Message-Id: <20171005.120629.2161199733119811102.davem@davemloft.net>
Date:   Thu, 05 Oct 2017 12:06:29 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     Michal.Kalderon@...ium.com
Cc:     netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        dledford@...hat.com, Ariel.Elior@...ium.com
Subject: Re: [PATCH v2 net-next 06/12] qed: Add LL2 slowpath handling

From: "Kalderon, Michal" <Michal.Kalderon@...ium.com>
Date: Thu, 5 Oct 2017 18:59:04 +0000

> From: Kalderon, Michal
> Sent: Tuesday, October 3, 2017 9:05 PM
> To: David Miller
>>From: David Miller <davem@...emloft.net>
>>Sent: Tuesday, October 3, 2017 8:17 PM
>>>> @@ -423,6 +423,41 @@ static void qed_ll2_rxq_parse_reg(struct qed_hwfn *p_hwfn,
>>>>  }
>>>>
>>>>  static int
>>>> +qed_ll2_handle_slowpath(struct qed_hwfn *p_hwfn,
>>>> +                     struct qed_ll2_info *p_ll2_conn,
>>>> +                     union core_rx_cqe_union *p_cqe,
>>>> +                     unsigned long *p_lock_flags)
>>>> +{
>>>...
>>>> +     spin_unlock_irqrestore(&p_rx->lock, *p_lock_flags);
>>>> +
>>>
>>>You can't drop this lock.
>>>
>>>Another thread can enter the loop of our caller and process RX queue
>>>entries, then we would return from here and try to process the same
>>>entries again.
>>
>>The lock is there to synchronize access to chains between qed_ll2_rxq_completion
>>and qed_ll2_post_rx_buffer. qed_ll2_rxq_completion can't be called from
>>different threads, the light l2 uses the single sp status block we have.
>>The reason we release the lock is to avoid a deadlock where as a result of calling
>>upper-layer driver it will potentially post additional rx-buffers.
> 
> Dave, is there anything else needed from me on this? 
> Noticed the series is still in "Changes Requested". 

I'm still not convinced that the lock dropping is legitimate.  What if a
spurious interrupt arrives?

If the execution path in the caller is serialized for some reason, why
are you using a spinlock and don't use that serialization for the mutual
exclusion necessary for these queue indexes?