linux-kernel - Re: [PATCH v5] can: mcp251xfd: fix infinite loop when xmit fails

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Message-ID: <20240515-athletic-sensible-swine-4e7692-mkl@pengutronix.de>
Date: Wed, 15 May 2024 13:12:37 +0200
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Vitor Soares <ivitro@...il.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>, 
	Thomas Kopp <thomas.kopp@...rochip.com>, Vincent Mailhol <mailhol.vincent@...adoo.fr>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Vitor Soares <vitor.soares@...adex.com>, linux-can@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v5] can: mcp251xfd: fix infinite loop when xmit fails

On 14.05.2024 15:34:01, Vitor Soares wrote:
> > > +void mcp251xfd_tx_obj_write_sync(struct work_struct *ws)
> > > +{
> > > +       struct mcp251xfd_priv *priv = container_of(ws, struct
> > > mcp251xfd_priv,
> > > +                                                  tx_work);
> > > +       struct mcp251xfd_tx_obj *tx_obj = priv->tx_work_obj;
> > > +       struct mcp251xfd_tx_ring *tx_ring = priv->tx;
> > > +       int err;
> > > +
> > > +       err = spi_sync(priv->spi, &tx_obj->msg);
> > > +       if (err)
> > > +               mcp251xfd_tx_failure_drop(priv, tx_ring, err);
> > > +
> > > +       priv->tx_work_obj = NULL;
> > 
> > Race condition:
> > - after spi_sync() the CAN frame is send
> > - after the TX complete IRQ the TX queue is restarted
> > - the xmit handler might get BUSY
> > - fill the tx_work_obj again

You can avoid the race condition by moving "priv->tx_work_obj = NULL;"
in front of the "spi_sync();". Right?

> > > +}
> > > +
> > >  static int mcp251xfd_tx_obj_write(const struct mcp251xfd_priv *priv,
> > >                                   struct mcp251xfd_tx_obj *tx_obj)
> > >  {
> > > @@ -175,7 +210,7 @@ netdev_tx_t mcp251xfd_start_xmit(struct sk_buff *skb,
> > >         if (can_dev_dropped_skb(ndev, skb))
> > >                 return NETDEV_TX_OK;
> > >  
> > > -       if (mcp251xfd_tx_busy(priv, tx_ring))
> > > +       if (mcp251xfd_tx_busy(priv, tx_ring) || priv->tx_work_obj)
> > 
> > This should not happen, but better save than sorry.
> 
> As there is the race condition you mentioned above, on this condition:
> priv->tx_work_obj = tx_obj --> xmit will return NETDEV_TX_BUSY
> 
> or
> 
> priv->tx_work_obj = NULL --> It goes through the rest of the code or
> the workqueue may sleep after setting tx_work_obj to NULL. Should I
> use work_busy() here instead or do you have another suggestion?

Yes, introduce mcp251xfd_work_busy().

I'm not sure what happens if the xmit is called between the
"priv->tx_work_obj = NULL" and the end of the work. Will queue_work()
return false, as the queue is still running?

> Everything else is clear to me and I'm addressing it for the v6.

Marc

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