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

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Date:   Wed, 27 Jan 2021 19:22:25 -0600
From:   Lijun Pan <lijunp213@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Lijun Pan <ljp@...ux.ibm.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net v2] ibmvnic: Ensure that CRQ entry read are correctly ordered

On Wed, Jan 27, 2021 at 7:06 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> 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?

The above 2 lines are the general description. The below 4 lines are
detailed explanations. If it is still confusing, we can delete the above
2 lines of comments.

>
> > +                      * 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;
> >               }
>