netdev - Re: [PATCH net-next] sh_eth: remove open coded netif

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Date:   Sat, 25 Mar 2023 23:56:09 +0300
From:   Sergey Shtylyov <s.shtylyov@....ru>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Wolfram Sang <wsa+renesas@...g-engineering.com>,
        <netdev@...r.kernel.org>, <linux-renesas-soc@...r.kernel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] sh_eth: remove open coded netif_running()

Hello!

On 3/23/23 11:32 AM, Geert Uytterhoeven wrote:
[...]
>>> It had a purpose back in the days, but today we have a handy helper.
>>
>>    Well, the is_opened flag doesn't get set/cleared at the same time as
>> __LINK_STATE_START. I'm not sure they are interchangeable...
>>
>>> Reported-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>> Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>
> 
>>> --- a/drivers/net/ethernet/renesas/sh_eth.c
>>> +++ b/drivers/net/ethernet/renesas/sh_eth.c
>>> @@ -2441,8 +2441,6 @@ static int sh_eth_open(struct net_device *ndev)
>>>
>>>       netif_start_queue(ndev);
>>>
>>> -     mdp->is_opened = 1;
>>> -
>>
>>    __LINK_STATE_START gets set before the ndo_open() method call, so
>> before the RPM call that enbales the clocks...
>>
>>>       return ret;
>>>
>>>  out_free_irq:
>>> @@ -2565,7 +2563,7 @@ static struct net_device_stats *sh_eth_get_stats(struct net_device *ndev)
>>>       if (mdp->cd->no_tx_cntrs)
>>>               return &ndev->stats;
>>>
>>> -     if (!mdp->is_opened)
>>> +     if (!netif_running(ndev))
>>>               return &ndev->stats;
>>
>>    I guess mdp->is_opened is checked here to avoid reading the counter
>> regs when RPM hasn't been called yet to enable the clocks...
> 
> Exactly, cfr. commit 7fa2955ff70ce453 ("sh_eth: Fix sleeping function
> called from invalid context").

   Yeah, pm_runtime_get_sync() couldn't be called in this case as
netstat_show() invoked read_lock() that ensued calling preempt_disable()...

> So you mean sh_eth_get_stats() can now be called after setting
> __LINK_STATE_START, but before RPM has enabled the clocks?

   Yes, probably...

> Is there some protection against parallel execution of ndo_open()
> and get_stats()?

   Haven't seen it (yet?)...

> Gr{oetje,eeting}s,
> 
>                         Geert

MBR, Sergey