netdev - Re: [PATCH net-next v7 1/8] bnxt_en: Add auxiliary driver support

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Date:   Tue, 17 Jan 2023 19:18:20 +0200
From:   Leon Romanovsky <leonro@...dia.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     Ajit Khaparde <ajit.khaparde@...adcom.com>,
        <andrew.gospodarek@...adcom.com>, <davem@...emloft.net>,
        <edumazet@...gle.com>, <jgg@...pe.ca>,
        <linux-kernel@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
        <michael.chan@...adcom.com>, <netdev@...r.kernel.org>,
        <pabeni@...hat.com>, <selvin.xavier@...adcom.com>
Subject: Re: [PATCH net-next v7 1/8] bnxt_en: Add auxiliary driver support

On Tue, Jan 17, 2023 at 02:31:01PM +0200, Leon Romanovsky wrote:
> On Mon, Jan 16, 2023 at 08:56:25PM -0800, Jakub Kicinski wrote:
> > On Sat, 14 Jan 2023 12:39:09 -0800 Ajit Khaparde wrote:
> > > > > +static void bnxt_aux_dev_release(struct device *dev)
> > > > > +{
> > > > > +     struct bnxt_aux_dev *bnxt_adev =
> > > > > +             container_of(dev, struct bnxt_aux_dev, aux_dev.dev);
> > > > > +     struct bnxt *bp = netdev_priv(bnxt_adev->edev->net);
> > > > > +
> > > > > +     bnxt_adev->edev->en_ops = NULL;
> > > > > +     kfree(bnxt_adev->edev);  
> > > >
> > > > And yet the reference counted "release" function accesses the bp->adev
> > > > like it must exist.
> > > >
> > > > This seems odd to me - why do we need refcounting on devices at all
> > > > if we can free them synchronously? To be clear - I'm not sure this is
> > > > wrong, just seems odd.  
> > > I followed the existing implementations in that regard. Thanks
> > 
> > Leon, could you take a look? Is there no problem in assuming bnxt_adev
> > is still around in the release function?
> 
> You caught a real bug. The auxdev idea is very simple - it needs to
> behave like driver core, but in the driver itself.

BTW, this can be classic example why assigning NULL pointers after
release is bad practice. It hides this class of errors.

+void bnxt_aux_dev_free(struct bnxt *bp)
+{
+       kfree(bp->aux_dev);
+       bp->aux_dev = NULL;
+}

Thanks