netdev - RE: [PATCH V3 3/6] vDPA: allow userspace to query features of a vDPA device

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Message-ID: <PH0PR12MB54813E8367D561ACFAAF3B99DC929@PH0PR12MB5481.namprd12.prod.outlook.com>
Date:   Sun, 24 Jul 2022 15:23:18 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>,
        Jason Wang <jasowang@...hat.com>,
        "mst@...hat.com" <mst@...hat.com>
CC:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xieyongji@...edance.com" <xieyongji@...edance.com>,
        "gautam.dawar@....com" <gautam.dawar@....com>
Subject: RE: [PATCH V3 3/6] vDPA: allow userspace to query features of a vDPA
 device


> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> Sent: Saturday, July 23, 2022 7:27 AM
> 
> On 7/6/2022 10:25 AM, Zhu, Lingshan wrote:
> >
> >
> > On 7/6/2022 1:01 AM, Parav Pandit wrote:
> >>> From: Zhu, Lingshan <lingshan.zhu@...el.com>
> >>> Sent: Tuesday, July 5, 2022 12:56 PM
> >>>> Both can be queried simultaneously. Each will return their own
> >>>> feature bits
> >>> using same attribute.
> >>>> It wont lead to the race.
> >>> How? It is just a piece of memory, xxxx[attr], do you see locks in
> >>> nla_put_u64_64bit()? It is a typical race condition, data accessed
> >>> by multiple producers / consumers.
> >> No. There is no race condition in here.
> >> And new attribute enum by no means avoid any race.
> >>
> >> Data put using nla_put cannot be accessed until they are transferred.
> > How this is guaranteed? Do you see errors when calling nla_put_xxx()
> > twice?
> Parav, did you miss this?
It is not called twice and reading attribute and packing in nla message is not race condition.