netdev - Re: [PATCH net v2] ibmvnic: Ensure that CRQ entry read are correctly ordered

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Message-ID: <20210127170105.011ebb9b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 27 Jan 2021 17:01:05 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Lijun Pan <ljp@...ux.ibm.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net v2] ibmvnic: Ensure that CRQ entry read are
 correctly ordered

On Mon, 25 Jan 2021 17:20:23 -0600 Lijun Pan wrote:
> Ensure that received Command-Response Queue (CRQ) entries are
> properly read in order by the driver. dma_rmb barrier has
> been added before accessing the CRQ descriptor to ensure
> the entire descriptor is read before processing.
> 
> Fixes: 032c5e82847a ("Driver for IBM System i/p VNIC protocol")
> Signed-off-by: Lijun Pan <ljp@...ux.ibm.com>
> ---
> v2: drop dma_wmb according to Jakub's opinion
> 
>  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 9778c83150f1..d84369bd5fc9 100644
> --- a/drivers/net/ethernet/ibm/ibmvnic.c
> +++ b/drivers/net/ethernet/ibm/ibmvnic.c
> @@ -5084,6 +5084,14 @@ static void ibmvnic_tasklet(struct tasklet_struct *t)
>  	while (!done) {
>  		/* Pull all the valid messages off the CRQ */
>  		while ((crq = ibmvnic_next_crq(adapter)) != NULL) {
> +			/* Ensure that the entire CRQ descriptor queue->msgs
> +			 * has been loaded before reading its contents.

I still find this sentence confusing, maybe you mean to say stored
instead of loaded?

> +			 * This barrier makes sure ibmvnic_next_crq()'s
> +			 * crq->generic.first & IBMVNIC_CRQ_CMD_RSP is loaded
> +			 * before ibmvnic_handle_crq()'s
> +			 * switch(gen_crq->first) and switch(gen_crq->cmd).

Yup, that makes perfect sense. It's about ordering of the loads. 

> +			 */
> +			dma_rmb();
>  			ibmvnic_handle_crq(crq, adapter);
>  			crq->generic.first = 0;
>  		}