Re: [PATCH net-next v3] net/smc: avoid atomic_set and smp_wmb in the tx path when possible

[Alexandra Winter <wintera@linux.ibm.com>]

On 20.11.23 10:11, Alexandra Winter wrote:
> 
> 
> On 20.11.23 04:20, Dust Li wrote:
>>> It seems to me that the purpose of conn->tx_pushing is
>>> a) Serve as a mutex, so only one thread per conn will call __smc_tx_sndbuf_nonempty().
>>> b) Repeat, in case some other thread has added data to sndbuf concurrently.
>>>
>>> I agree that this patch does not change the behaviour of this function and removes an
>>> atomic_set() in the likely path.
>>>
>>> I wonder however: All callers of smc_tx_sndbuf_nonempty() must hold the socket lock.
>>> So how can we ever run in a concurrency situation?
>>> Is this handling of conn->tx_pushing necessary at all?
>> Hi Sandy,
>>
>> Overall, I think you are right. But there is something we need to take care.
>>
>> Before commit 6b88af839d20 ("net/smc: don't send in the BH context if
>> sock_owned_by_user"), we used to call smc_tx_pending() in the soft IRQ,
>> without checking sock_owned_by_user(), which would caused a race condition
>> because bh_lock_sock() did not honor sock_lock(). To address this issue,
>> I have added the tx_pushing mechanism. However, with commit 6b88af839d20,
>> we now defer the transmission if sock_lock() is held by the user.
>> Therefore, there should no longer be a race condition. Nevertheless, if
>> we remove the tx_pending mechanism, we must always remember not to call
>> smc_tx_sndbuf_nonempty() in the soft IRQ when the user holds the sock lock.
>>
>> Thanks
>> Dust
> 
> 
> ok, I understand.
> So whoever is willing to give it a try and simplify smc_tx_sndbuf_nonempty(),
> should remember to document that requirement/precondition.
> Maybe in a Function context section of a kernel-doc function decription?
> (as described in https://docs.kernel.org/doc-guide/kernel-doc.html)
> Although smc_tx_sndbuf_nonempty() is not exported, this format is helpful.
> 


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