netdev - Re: [PATCH net] net: davicom: dm9000: dm9000_phy_write(): fix deadlock during netdev watchdog handling

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Message-ID: <20231011-said-hemlock-834e5698a7a3-mkl@pengutronix.de>
Date: Wed, 11 Oct 2023 08:43:31 +0200
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Francois Romieu <romieu@...zoreil.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Kalle Valo <kvalo@...nel.org>, Wei Fang <wei.fang@....com>,
	kernel@...gutronix.de, stable@...r.kernel.org
Subject: Re: [PATCH net] net: davicom: dm9000: dm9000_phy_write(): fix
 deadlock during netdev watchdog handling

On 11.10.2023 00:21:31, Francois Romieu wrote:
> Marc Kleine-Budde <mkl@...gutronix.de> :
> > The dm9000 takes the db->lock spin lock in dm9000_timeout() and calls
> > into dm9000_init_dm9000(). For the DM9000B the PHY is reset with
> > dm9000_phy_write(). That function again takes the db->lock spin lock,
> > which results in a deadlock. For reference the backtrace:
> [...]
> > To workaround similar problem (take mutex inside spin lock ) , a
> > "in_timeout" variable was added in 582379839bbd ("dm9000: avoid
> > sleeping in dm9000_timeout callback"). Use this variable and not take
> > the spin lock inside dm9000_phy_write() if in_timeout is true.
> > 
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Marc Kleine-Budde <mkl@...gutronix.de>
> > ---
> > During the netdev watchdog handling the dm9000 driver takes the same
> > spin lock twice. Avoid this by extending an existing workaround.
> > ---
> 
> I can review it but I can't really endorse it. :o)
> 
> Extending ugly workaround in pre-2000 style device drivers...
> I'd rather see the thing fixed if there is some real use for it.

There definitely are still users of this drivers on modern kernels out
there.

I too don't like the feeling of wrapping more and more duct tape
around existing drivers. How about moving the functionality to
dm9000_phy_write_locked() and leave the locking in dm9000_phy_write().
I will prepare a patch.

regards,
Marc

-- 
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