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

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Message-ID: <87o8jmyosh.fsf@mpe.ellerman.id.au>
Date:   Wed, 25 Nov 2020 16:43:26 +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,
        tlfalcon@...ux.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:
> 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.

>  		next = ibmvnic_next_scrq(adapter, adapter->rx_scrq[scrq_num]);
>  		rx_buff =
>  		    (struct ibmvnic_rx_buff *)be64_to_cpu(next->
> @@ -3098,6 +3100,9 @@ static int ibmvnic_complete_tx(struct ibmvnic_adapter *adapter,
>  		unsigned int pool = scrq->pool_index;
>  		int num_entries = 0;
>  
> +		/* ensure that the correct descriptor entry is read */
> +		dma_rmb();
> +
>  		next = ibmvnic_next_scrq(adapter, scrq);
>  		for (i = 0; i < next->tx_comp.num_comps; i++) {
>  			if (next->tx_comp.rcs[i]) {
> @@ -3498,6 +3503,9 @@ static union sub_crq *ibmvnic_next_scrq(struct ibmvnic_adapter *adapter,
>  	}
>  	spin_unlock_irqrestore(&scrq->lock, flags);
>  
> +	/* ensure that the entire SCRQ descriptor is read */
> +	dma_rmb();
> +
>  	return entry;
>  }

cheers