netdev - RE: [PATCH net v4] ravb: Fix races between ravb_tx_timeout

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Date: Mon, 27 Nov 2023 23:58:23 +0000
From: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To: Sergey Shtylyov <s.shtylyov@....ru>, "davem@...emloft.net"
	<davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
	"kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com" <pabeni@...hat.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>
Subject: RE: [PATCH net v4] ravb: Fix races between ravb_tx_timeout_work() and
 net related ops

Hi Sergey,

> From: Sergey Shtylyov, Sent: Tuesday, November 28, 2023 1:28 AM
> 
> On 11/27/23 3:24 PM, Yoshihiro Shimoda wrote:
> 
> > Fix races between ravb_tx_timeout_work() and functions of net_device_ops
> > and ethtool_ops by using rtnl_trylock() and rtnl_unlock(). Note that
> > since ravb_close() is under the rtnl lock and calls cancel_work_sync(),
> > ravb_tx_timeout_work() should calls rtnl_trylock(). Otherwise, a deadlock
> > may happen in ravb_tx_timeout_work() like below:
> >
> > CPU0			CPU1
> > 			ravb_tx_timeout()
> > 			schedule_work()
> > ...
> > __dev_close_many()
> > // Under rtnl lock
> > ravb_close()
> > cancel_work_sync()
> > // Waiting
> > 			ravb_tx_timeout_work()
> > 			rtnl_lock()
> > 			// This is possible to cause a deadlock
> >
> > If rtnl_trylock() fails, rescheduling the work with sleep for 1 msec.
> >
> > Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
> > Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
> 
> Reviewed-by: Sergey Shtylyov <s.shtylyov@....ru>

Thank you very much for your review!

> [...]
> 
> > diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> > index c70cff80cc99..7c007ecd3ff6 100644
> > --- a/drivers/net/ethernet/renesas/ravb_main.c
> > +++ b/drivers/net/ethernet/renesas/ravb_main.c
> > @@ -1874,6 +1874,12 @@ static void ravb_tx_timeout_work(struct work_struct *work)
> >  	struct net_device *ndev = priv->ndev;
> >  	int error;
> >
> > +	if (!rtnl_trylock()) {
> > +		usleep_range(1000, 2000);
> 
>    Why not msleep(1) though?

It's for the following guideline:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/timers/timers-howto.rst?h=v6.7-rc3
-----
	SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
		* Use usleep_range
-----

Best regards,
Yoshihiro Shimoda

> > +		schedule_work(&priv->work);
> > +		return;
> > +	}
> > +
> >  	netif_tx_stop_all_queues(ndev);
> >
> >  	/* Stop PTP Clock driver */
> [...]
> 
> MBR, Sergey