netdev - RE: [PATCH] net: renesas: sh_eth: Fix freeing wrong tx descriptor

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Date:   Wed, 8 Sep 2021 05:45:01 +0000
From:   Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:     Sergey Shtylyov <s.shtylyov@....ru>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>
Subject: RE: [PATCH] net: renesas: sh_eth: Fix freeing wrong tx descriptor

Hi Sergey,

> From: Sergey Shtylyov, Sent: Wednesday, September 8, 2021 4:30 AM
> 
> On 9/7/21 2:29 PM, Yoshihiro Shimoda wrote:
> 
> > The cur_tx counter must be incremented after TACT bit of
> > txdesc->status was set. However, a CPU is possible to reorder
> > instructions and/or memory accesses between cur_tx and
> > txdesc->status. And then, if TX interrupt happened at such a
> > timing, the sh_eth_tx_free() may free the descriptor wrongly.
> > So, add wmb() before cur_tx++.
> 
>    Not dma_wmb()? :-)

On armv8, dma_wmb() is DMB OSHST, and wmb() is DSB ST.
IIUC, DMB OSHST is not affected the ordering of instructions.
So, we have to use wmb().

> > Otherwise NETDEV WATCHDOG timeout is possible to happen.
> >
> > Fixes: 86a74ff21a7a ("net: sh_eth: add support for Renesas SuperH Ethernet")
> > Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
> 
> Reviewed-by: Sergey Shtylyov <s.shtylyov@....ru>

Thank you for your review!

Best regards,
Yoshihiro Shimoda