linux-kernel - Re: [PATCH] net: macb: Increment rx bd head after allocating skb and buffer

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Message-ID: <CAFcVECJuB-F8=Gh3=jQsGhWwnNK2=-KvzzQ4MQQGCPoypiSXOQ@mail.gmail.com>
Date:   Tue, 10 May 2022 19:27:22 +0530
From:   Harini Katakam <harinik@...inx.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Harini Katakam <harini.katakam@...inx.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        David Miller <davem@...emloft.net>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        Jakub Kicinski <kuba@...nel.org>, dumazet@...gle.com,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Simek <michal.simek@...inx.com>,
        Radhey Shyam Pandey <radhey.shyam.pandey@...inx.com>
Subject: Re: [PATCH] net: macb: Increment rx bd head after allocating skb and buffer

Hi Paolo,

On Tue, May 10, 2022 at 6:54 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hello,
>
> On Mon, 2022-05-09 at 17:49 +0530, Harini Katakam wrote:
> > In gem_rx_refill rx_prepared_head is incremented at the beginning of
> > the while loop preparing the skb and data buffers. If the skb or data
> > buffer allocation fails, this BD will be unusable BDs until the head
> > loops back to the same BD (and obviously buffer allocation succeeds).
> > In the unlikely event that there's a string of allocation failures,
> > there will be an equal number of unusable BDs and an inconsistent RX
> > BD chain. Hence increment the head at the end of the while loop to be
> > clean.
> >
> > Signed-off-by: Harini Katakam <harini.katakam@...inx.com>
> > Signed-off-by: Michal Simek <michal.simek@...inx.com>
> > Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@...inx.com>
>
> This looks like targeting the "net" tree, please repost adding a
> suitable Fixes tag.

Thanks for the review. This behavior would theoretically have been the
same since
GEM RX path handling was introduced ~9 yrs ago but I'm not sure since I cannot
reproduce. Probably this?
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/net/ethernet/cadence?id=4df95131ea803bcb94f472d465c73ed57015c470
Also, this patch can't be backported to stable branches so far back
since the driver
files have changed. It can also be queued for net-next instead.

Regards,
Harini