netdev - Re: [PATCH v5 4/7] can: Add Nuvoton NCT6694 CAN support

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Message-ID: <6e349f0f-6509-4a3b-bb75-e2381e9205c6@wanadoo.fr>
Date: Wed, 15 Jan 2025 00:11:41 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Ming Yu <a0282524688@...il.com>
Cc: tmyu0@...oton.com, lee@...nel.org, linus.walleij@...aro.org,
 brgl@...ev.pl, andi.shyti@...nel.org, mkl@...gutronix.de,
 andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
 kuba@...nel.org, pabeni@...hat.com, wim@...ux-watchdog.org,
 linux@...ck-us.net, jdelvare@...e.com, alexandre.belloni@...tlin.com,
 linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
 linux-i2c@...r.kernel.org, linux-can@...r.kernel.org,
 netdev@...r.kernel.org, linux-watchdog@...r.kernel.org,
 linux-hwmon@...r.kernel.org, linux-rtc@...r.kernel.org,
 linux-usb@...r.kernel.org
Subject: Re: [PATCH v5 4/7] can: Add Nuvoton NCT6694 CAN support

On 14/01/2025 at 19:46, Ming Yu wrote:
> Dear Vincent,
> 
> Thank you for your reply,
> I'll add comments to describe these locks in the next patch,
> 
> Vincent Mailhol <mailhol.vincent@...adoo.fr> 於 2025年1月14日 週二 下午4:06寫道：

(...)

>>> +static int nct6694_can_get_berr_counter(const struct net_device *ndev,
>>> +                                       struct can_berr_counter *bec)
>>> +{
>>> +       struct nct6694_can_priv *priv = netdev_priv(ndev);
>>> +       struct nct6694_can_event *evt = priv->rx->event;
>>> +       struct nct6694_cmd_header cmd_hd;
>>> +       u8 mask = NCT6694_CAN_EVENT_REC | NCT6694_CAN_EVENT_TEC;
>>> +       int ret;
>>> +
>>> +       guard(mutex)(&priv->lock);
>>> +
>>> +       cmd_hd = (struct nct6694_cmd_header) {
>>> +               .mod = NCT6694_CAN_MOD,
>>> +               .cmd = NCT6694_CAN_EVENT,
>>> +               .sel = NCT6694_CAN_EVENT_SEL(priv->can_idx, mask),
>>> +               .len = cpu_to_le16(sizeof(priv->rx->event))
>>> +       };
>>> +
>>> +       ret = nct6694_read_msg(priv->nct6694, &cmd_hd, evt);
>>> +       if (ret < 0)
>>> +               return ret;
>>
>> You are holding the priv->lock mutex before calling
>> nct6694_read_msg(). But nct6694_read_msg() then holds the
>> nct6694->access_lock mutex. Why do you need a double mutex here? What
>> kind of race scenario are you trying to prevent here?
>>
> 
> I think priv->lock need to be placed here to prevent priv->rx from
> being assigned by other functions, and nct6694->access_lock ensures
> that the nct6694_read_msg() transaction is completed.
> But in this case, cmd_hd does not need to be in priv->lock's scope.

So, the only reason for holding priv->lock is because priv->rx is shared
between functions.

struct nct6694_can_event is only 8 bytes. And you only need it for the
life time of the function so it can simply be declared on the stack:

  	struct nct6694_can_event evt;

and with this, no more need to hold the lock. And the same thing also
applies to the other functions.

Here, by trying to optimize the memory for only a few bytes, you are
getting a huge penalty on the performance by putting locks on all the
functions. This is not a good tradeoff.

>>> +       bec->rxerr = evt[priv->can_idx].rec;
>>> +       bec->txerr = evt[priv->can_idx].tec;
>>> +
>>> +       return 0;
>>> +}


Yours sincerely,
Vincent Mailhol