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

Ming Yu
2025-01-15T02:11:44Z

Vincent Mailhol <mailhol.vincent@wanadoo.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