linux-kernel - Re: [PATCH net v3 2/2] net: ethernet: ti: am65-cpsw: avoid devm_alloc

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Message-ID: <s5000qsr-8nps-87os-np52-oqq6643o35o2@syhkavp.arg>
Date: Fri, 4 Oct 2024 11:37:20 -0400 (EDT)
From: Nicolas Pitre <nico@...xnic.net>
To: Roger Quadros <rogerq@...nel.org>
cc: "David S. Miller" <davem@...emloft.net>, 
    Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
    Paolo Abeni <pabeni@...hat.com>, 
    Grygorii Strashko <grygorii.strashko@...com>, 
    Vignesh Raghavendra <vigneshr@...com>, netdev@...r.kernel.org, 
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v3 2/2] net: ethernet: ti: am65-cpsw: avoid
 devm_alloc_etherdev, fix module removal

On Fri, 4 Oct 2024, Roger Quadros wrote:

> Hi Nicolas,
> 
> On 04/10/2024 07:10, Nicolas Pitre wrote:
> > From: Nicolas Pitre <npitre@...libre.com>
> > 
> > Usage of devm_alloc_etherdev_mqs() conflicts with
> > am65_cpsw_nuss_cleanup_ndev() as the same struct net_device instances
> > get unregistered twice. Switch to alloc_etherdev_mqs() and make sure
> 
> Do we know why the same net device gets unregistered twice?

When using devm_alloc_etherdev_mqs() every successful allocation is put 
in a resource list tied to the device. When the driver is removed, 
there's a net device unregister from am65_cpsw_nuss_cleanup_ndev() and 
another one from devm_free_netdev().

We established in patch #1 that net devices must be unregistered before 
devlink_port_unregister() is invoked, meaning we can't rely on the 
implicit devm_free_netdev() as it happens too late, hence the explicit 
am65_cpsw_nuss_cleanup_ndev().

> > am65_cpsw_nuss_cleanup_ndev() unregisters and frees those net_device
> > instances properly.
> > 
> > With this, it is finally possible to rmmod the driver without oopsing
> > the kernel.
> > 
> > Fixes: 93a76530316a ("net: ethernet: ti: introduce am65x/j721e gigabit eth subsystem driver")
> > Signed-off-by: Nicolas Pitre <npitre@...libre.com>
> > ---
> >  drivers/net/ethernet/ti/am65-cpsw-nuss.c | 20 ++++++++++++--------
> >  1 file changed, 12 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/ti/am65-cpsw-nuss.c b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> > index f6bc8a4dc6..e95457c988 100644
> > --- a/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> > +++ b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
> > @@ -2744,10 +2744,9 @@ am65_cpsw_nuss_init_port_ndev(struct am65_cpsw_common *common, u32 port_idx)
> >  		return 0;
> >  
> >  	/* alloc netdev */
> > -	port->ndev = devm_alloc_etherdev_mqs(common->dev,
> > -					     sizeof(struct am65_cpsw_ndev_priv),
> > -					     AM65_CPSW_MAX_QUEUES,
> > -					     AM65_CPSW_MAX_QUEUES);
> > +	port->ndev = alloc_etherdev_mqs(sizeof(struct am65_cpsw_ndev_priv),
> > +					AM65_CPSW_MAX_QUEUES,
> > +					AM65_CPSW_MAX_QUEUES);
> 
> Can we solve this issue without doing this change as
> there are many error cases relying on devm managed freeing of netdev.

If you know of a way to do this differently I'm all ears.

About the many error cases needing the freeing of net devices, as far as 
I know they're all covered with this patch.

> I still can't see what we are doing wrong in existing code.

Did you try to rmmod this driver lately?


Nicolas