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

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Message-ID: <CAOoeyxVST6rEqp65rU6ZgmM-rSkAdeUVM=0nTLZYrqiO4DbQOA@mail.gmail.com>
Date: Wed, 15 Jan 2025 10:11:32 +0800
From: Ming Yu <a0282524688@...il.com>
To: Vincent Mailhol <mailhol.vincent@...adoo.fr>
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

Vincent Mailhol <mailhol.vincent@...adoo.fr> 於 2025年1月14日 週二 下午11:12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.
>

Since nct6694_read_msg()/nct6694_write_msg() process URBs via
usb_bulk_msg(), the transferred data must not be located on the stack.
For more details about allocating buffers for transmitting data,
please refer to the link:
https://lore.kernel.org/linux-can/20241028-observant-gentle-doberman-0a2baa-mkl@pengutronix.de/

Thanks,
Ming