netdev - Re: [PATCH net 1/2] ibmvnic: Ensure that SCRQ entry reads are correctly ordered

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Date:   Fri, 27 Nov 2020 10:57:39 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Thomas Falcon <tlfalcon@...ux.ibm.com>, netdev@...r.kernel.org
Cc:     cforno12@...ux.ibm.com, ljp@...ux.vnet.ibm.com,
        ricklind@...ux.ibm.com, dnbanerg@...ibm.com,
        drt@...ux.vnet.ibm.com, brking@...ux.vnet.ibm.com,
        sukadev@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH net 1/2] ibmvnic: Ensure that SCRQ entry reads are correctly ordered

Thomas Falcon <tlfalcon@...ux.ibm.com> writes:
> On 11/24/20 11:43 PM, Michael Ellerman wrote:
>> Thomas Falcon <tlfalcon@...ux.ibm.com> writes:
>>> Ensure that received Subordinate Command-Response Queue (SCRQ)
>>> entries are properly read in order by the driver. These queues
>>> are used in the ibmvnic device to process RX buffer and TX completion
>>> descriptors. dma_rmb barriers have been added after checking for a
>>> pending descriptor to ensure the correct descriptor entry is checked
>>> and after reading the SCRQ descriptor to ensure the entire
>>> descriptor is read before processing.
>>>
>>> Fixes: 032c5e828 ("Driver for IBM System i/p VNIC protocol")
>>> Signed-off-by: Thomas Falcon <tlfalcon@...ux.ibm.com>
>>> ---
>>>   drivers/net/ethernet/ibm/ibmvnic.c | 8 ++++++++
>>>   1 file changed, 8 insertions(+)
>>>
>>> diff --git a/drivers/net/ethernet/ibm/ibmvnic.c b/drivers/net/ethernet/ibm/ibmvnic.c
>>> index 2aa40b2..489ed5e 100644
>>> --- a/drivers/net/ethernet/ibm/ibmvnic.c
>>> +++ b/drivers/net/ethernet/ibm/ibmvnic.c
>>> @@ -2403,6 +2403,8 @@ static int ibmvnic_poll(struct napi_struct *napi, int budget)
>>>   
>>>   		if (!pending_scrq(adapter, adapter->rx_scrq[scrq_num]))
>>>   			break;
>>> +		/* ensure that we do not prematurely exit the polling loop */
>>> +		dma_rmb();
>> I'd be happier if these comments were more specific about which read(s)
>> they are ordering vs which other read(s).
>>
>> I'm sure it's obvious to you, but it may not be to a future author,
>> and/or after the code has been refactored over time.
>
> Thank you for reviewing! I will submit a v2 soon with clearer comments 
> on the reads being ordered here.

Thanks.

cheers