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Date:   Fri, 6 Aug 2021 11:25:23 +0200
From:   Marc Kleine-Budde <mkl@pengutronix.de>
To:     Dario Binacchi <dariobin@libero.it>
Cc:     linux-kernel@vger.kernel.org,
        Gianluca Falavigna <gianluca.falavigna@inwind.it>,
        Andrew Lunn <andrew@lunn.ch>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Oliver Hartkopp <socketcan@hartkopp.net>,
        Vincent Mailhol <mailhol.vincent@wanadoo.fr>,
        Wolfgang Grandegger <wg@grandegger.com>,
        linux-can@vger.kernel.org, netdev@vger.kernel.org
Subject: Re: [RESEND PATCH 4/4] can: c_can: cache frames to operate as a true
 FIFO
Message-ID: <20210806092523.hij5ejjq6wecbgfr@pengutronix.de>
References: <20210725161150.11801-1-dariobin@libero.it>
 <20210725161150.11801-5-dariobin@libero.it>
 <20210804094515.ariv7d24t2i4hic5@pengutronix.de>
 <1485600069.218377.1628194566441@mail1.libero.it>
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On 05.08.2021 22:16:06, Dario Binacchi wrote:
> > > --- a/drivers/net/can/c_can/c_can.h
> > > +++ b/drivers/net/can/c_can/c_can.h
> > > @@ -200,6 +200,7 @@ struct c_can_priv {
> > >  	atomic_t sie_pending;
> > >  	unsigned long tx_dir;
> > >  	int last_status;
> > > +	spinlock_t tx_lock;
> >=20
> > What does the spin lock protect?
[...]
> > > @@ -483,7 +469,11 @@ static netdev_tx_t c_can_start_xmit(struct sk_bu=
ff *skb,
> > >  	if (c_can_get_tx_free(tx_ring) =3D=3D 0)
> > >  		netif_stop_queue(dev);
> > > =20
> > > -	obj =3D idx + priv->msg_obj_tx_first;
> > > +	spin_lock_bh(&priv->tx_lock);
> >=20
> > What does the spin_lock protect? The ndo_start_xmit function is properly
> > serialized by the networking core.
> >=20
>=20
> The spin_lock protects the access to the IF_TX interface.

How? You only use the spin_lock in c_can_start_xmit(), but not anywhere
else.

> Enabling the transmission of cached messages occur inside interrupt

The call chain is c_can_poll() -> c_can_do_tx(), and c_can_poll() is
called from NAPI, which is not the IRQ handler.

> and the use of the IF_RX interface, which would avoid the use of the
> spinlock, has not been validated by the tests.

What do you mean be has not been validated?

The driver already uses IF_RX to avoid concurrent access in
c_can_do_tx() for c_can_inval_tx_object() [1], why not use IF_RX for
c_can_object_put(), too?

[1] https://lore.kernel.org/r/20210302215435.18286-4-dariobin@libero.it

Marc

--=20
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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