netdev - Re: atl1c drivers run 'napi/eth%d-385' named threads with unsubstituted %d

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Message-ID: <YetFsbw6885xUwSg@lunn.ch>
Date:   Sat, 22 Jan 2022 00:45:53 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Sergei Trofimovich <slyich@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: atl1c drivers run 'napi/eth%d-385' named threads with
 unsubstituted %d

On Fri, Jan 21, 2022 at 09:57:47PM +0000, Sergei Trofimovich wrote:
> Hia atl1c maintainers!
> 
> This cosmetics bothered me for some time: atl1c driver
> shows unexpanded % in kernel thread names. Looks like a
> minor bug:
> 
>     $ ping -f 172.16.0.1  # host1
>     $ top  # host2
>     ...
>     621 root 20 0 0 0 0 S 11.0 0.0 0:05.01 napi/eth%d-385
>     622 root 20 0 0 0 0 S  5.6 0.0 0:02.64 napi/eth%d-386
>     ...
> 
> Was happening for a few years. Likely not a recent regression.
> 
> System:
> - linux-5.16.1
> - x86_64
> - 02:00.0 Ethernet controller: Qualcomm Atheros AR8151 v2.0 Gigabit Ethernet (rev c0)
> 
> >From what I understand thread name comes from somewhere around:
> 
>   net/core/dev.c:
>     int dev_set_threaded(struct net_device *dev, bool threaded)
>     ...
>         err = napi_kthread_create(napi);
>     ...
>     static int napi_kthread_create(struct napi_struct *n)
>     ...
>         n->thread = kthread_run(napi_threaded_poll, n, "napi/%s-%d",
> 
>   drivers/net/ethernet/atheros/atl1c/atl1c_main.c:
>     static int atl1c_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
>     ...
>         dev_set_threaded(netdev, true);
> 
>   ${somewhere} (not sure where):
>     ...
>       strcpy(netdev->name, "eth%d");
> 
> I was not able to pinpoint where expansion should ideally happen.
> Looks like many driver do `strcpy(netdev->name, "eth%d");` style
> initialization and almost none call `dev_set_threaded(netdev, true);`.
> 
> Can you help me find it out how it should be fixed?

Hi Sergei

This is a fun one.

So, the driver does the usual alloc_etherdev_mq()

https://elixir.bootlin.com/linux/latest/source/drivers/net/ethernet/atheros/atl1c/atl1c_main.c#L2703

which ends up here:

https://elixir.bootlin.com/linux/latest/source/net/ethernet/eth.c#L391

struct net_device *alloc_etherdev_mqs(int sizeof_priv, unsigned int txqs,
                                      unsigned int rxqs)
{
        return alloc_netdev_mqs(sizeof_priv, "eth%d", NET_NAME_UNKNOWN,
                                ether_setup, txqs, rxqs);
}

So at this point in time, the device has the name "eth%d".

The normal flow is that sometime later in probe, it calls
register_netdev().

https://elixir.bootlin.com/linux/latest/source/net/core/dev.c#L10454

if you follow that down, you get to: __dev_alloc_name(), which does
the expansion of the %d to an actual number:

https://elixir.bootlin.com/linux/latest/source/net/core/dev.c#L1087

So between alloc_etherdev_mq() and register_netdev(), the device name
is not valid. And as you pointed out, dev_set_threaded() tries to use
the name, and is called between these two.

The atl1c driver appears to be the only driver actually doing
this. There is a sysfs interface which can call dev_set_threaded(),
but the sysfs interface is probably not available until after
register_netdev() has given the interface its name.

There is a fix for atl1c. Any time after alloc_etherdev_mq(), the
driver can call dev_alloc_name().

So please give this a try. I've not even compile tested it...

iff --git a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
index da595242bc13..983a52f77bda 100644
--- a/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
+++ b/drivers/net/ethernet/atheros/atl1c/atl1c_main.c
@@ -2706,6 +2706,10 @@ static int atl1c_probe(struct pci_dev *pdev, const struct pci_device_id *ent)
                goto err_alloc_etherdev;
        }
 
+       err = dev_alloc_name(netdev, netdev->name);
+       if (err < 0)
+               goto err_init_netdev;
+
        err = atl1c_init_netdev(netdev, pdev);
        if (err) {
                dev_err(&pdev->dev, "init netdevice failed\n");

If this works, i can turn it into a real patch submission.

   Andrew